Hey how do you get Dialga can some 1 tell me the complete walkthrough?

I'll start after the Team Galactic HQ. Head to Mount Corenet. You will need strength, surf, rock climb, tons of balls (ultra, timer, and Dusk if you play at night.), and a ton of medical items. Since Team Galactic is using the ledendary Dialga to complete their scheme, the area is littered with grunts. I suggest you bring Pokemon Lv. 43 or higher. Just follow the path of the Grunts. This will take you outside. You will need rock climb to scale the mountainside. Once you reenter the cave, continue to follow the path of grunts. This will lead you to Spear Pillar. Save because you do not want to climb the mountain again if Team Galactic wins. You will have a double battle with Mars and Venus. Luckly, your rival shows for revenge and assist you in the fight. With the Admins defeated, your rival will heal you and depart. Cyrus has ressurected Dialga before your first fight and The three Pokemon of the lake intervene. With his plans fallen apart, The leader takes his anger out on you. After that rough battle, remember the healing items I suggested, use them since you must catch Dialga before you can leave. He will be Lv 47 when you fight him. Unlike other ledgendaries, He is not a complete pain to catch. (I only caught him with my 1st Ultra ball, a feat I have never done with a ledendary before.) After you catch him (or defeat him by mistake) Professor Rowan and his assistant will take you to the exit. Before you leave, go past the place where you fought Dialga and grab the Adamant orb. It is his exclusive Item that powers his Steel and Dragon attacks.

If you want a walkthrough, then why not just look at the walkthrough sections on this site, or on others? Anyway...

First, obtain 6 badges. From here, it gets a little confusing. You'll meet up with your rival to go to the library. Heal up, first, and do so. Talk to Prof. Rowan and he'll tell you about the three pixies of Sinnoh [Mesprit, Uxie, and Azelf]. Then, there will be an earthquake, which is from an explosion at Lake Valor. So, fly to Pastoria and go there. Fight the commander and she'll tell you to go to Lake Verity. Here, beat the commander there and she'll go off. Prof. Rowan will ask you to see your rival at Lake Acuity, near Snowpoint City. At this point, you'll want a team of Pokemon that know: HM03 Surf, HM04 Strength, HM05 Defog, and HM06 Rock Smash. Also bring a bunch of repel [preferably max repel, if you don't want to run into any pokemon. Also, but some potions and revives JUST IN CASE.
Now, you're going to have to start at Celestic Town/Eterna City, wherever you want. If possible, start at Celestic City. Go in the Mt. Coronet entrance from there, and go all the way to the right [if you started at Eterna City], or if you started at Celestic Town, just go up and use strength on that boulder. Try to access the TM and Rare Candy to the west at some point. Otherwise, just keep pushing the boulder up and go down the stairs. Use Defog in this room, with some boulders to smash, and go down the stairs again. In here, you'll need to use Defog again, and you can also use rock smash, strength, and surf if you'll notice the big lake. Once you have the national dex and obtained the super rod, you can use the super rod in this room in the water to possibly catch Feebas. Moving along, you can pick the items you run into in this room, but your main goal is at the end, to go up the stairs and go around and exit the cave.
You're now on route 216, and there's not much you can do for now. Battle a few trainers and you'll come across a house where you can rest and heal your pokemon. THIS IS VERY IMPORTANT TO KEEP IN YOUR MIND! Train your Pokemon here, as it is a decent training place and you can always train your Pokemon in that house. Otherwise, move north to route 217. There are quite a few trainers to take note of, as well as the required item HM08 Rock Climb. You'll find this to the northwest corner of this route, near a house. Once you've obtained this item, go in the house and talk to the hiker to obtain the Icicle Plate. There are also 2 more items on this route, but I don't remember what they were called. So, just move along to the Lake Acuity Lakefront, to find out that it's blocked for now. So, just move along to Snowpoint City. There's nothing to do here, so just heal up and buy some items if necessary. Then, move along to the 7th Pokemon gym. It's not that confusing, just try to take out all the snowballs and go down the straight path that leads to the 7th gym leader. If you have a fire Pokemon in your team, GREAT! Use the fire Pokemon against the gym leader, and when you have the 7th badge, heal up, and go back to Lake Acuity Lakefront. Go into the Lake Acuity, as it is accessible now, and see your rival miserably lose. From here, fly to Veilstone City. Go in front of their HQ and talk to the clueless Galactic grunt. He'll fly off, leaving behind a Storage Key. With this, go into their warehouse [opposite side of town] and in here, you'll eventually find the Galactic Key. With this, go BACK to their HQ and go in it. Open the sliding door and maneuver through a series of teleporters until you reach their leader, Cyrus. Beat him, and go to their "lab". Beat the commander there, and release the three pixies.
Thought it was over? Almost! Now, fly to Oreburgh City and buy a bunch of revives, full restores, and max repels, and make sure you have Pokemon in your team that know HM03 Surf, HM06 Rock Smash, and HM08 Rock Climb. If possible, have a Pokemon know HM04 Strength to save you from wild Pokemon in a few situations. Also, bring at least 23 Ultra Balls, and if you're doing this at night, bring at least 30 Dusk Balls. I reccomend you do this at night, and buy 30 Dusk Balls.
Now, go north and go all way east and go in the Mt. Coronet entrance from here.

"Then, use Surf to cross the Northern body of water and then use Rock Climb to climb the rocks.

Upstairs, you can find a Max Potion on your way to the west up the stairs,
Along with an Ether in the nearby rock. An Escape Rope is on the very south
Side of the continued path, although you'll want to take the stairs going up
Onto the elevated area. There's an exit in the southwest corner of the room
That will take you outside to the west side of Route 207, where you can use
Rock Climb to access an Iron. TM80 (Rock Slide) can be found by crossing the
Bridge, then taking the southernly route to reach it. A Revive can be found by
Taking the northern route instead. Near the very back of the cave, you'll find an opening in the wall (however, if you have not done the events in the Veilstone Galactic Building and are coming here for loot immediately after getting Rock Climb, this opening will be sealed with depictions of the Legendary Trio, similar to the gate in Celestic Town). Enter it, and then make your way up the stairs.

The stairs to the right lead to a hidden Max Revive inside of the southern
Rock. Otherwise, just keep heading north. Fight your way through the two Grunts along your way to the peak of Mt. Coronet.

As you enter, you'll see a familiar sight: snow. Blinding snow. Like the
Routes on your way to Snowpoint City, it is snowing at the peak of Mt.
Coronet, and the weather will be Hail inside of each battle.

This section sometimes confuses people, because there are lots of areas to
Explore on the peak and I like to cover them all, so I'm going to list here
A quick-'n-easy path to take to get straight to business:

1. Head east, north, and use Rock Climb.
2. Go west, south, a bit more west, and head north into the cave.
3. Use Rock Climb and follow the path in the cave east to the exit.
4. Climb down the stairs in the peak and then use Rock Climb in the chasm
to climb the western rocks.
5. Keep heading west until you get to a cave.
6. It's straightforward from here on in, so just keep following the path
and you'll get there."

--Marriland's Walkthrough on GameFAQs.

NOTE: Heal up right after you exit, and make sure it is night and you have at least 30 Dusk Balls. Then, SAVE NOW.

Continue to fight Mars and Jupiter, and your rival will come up to help you fight. Win, and your rival will heal you up. Now, battle Cyrus.
Beat him, and HEAL UP NOW! REMEMBER TO HAVE AT LEAST 30 DUSK BALLS. I CAN'T STRESS THAT ENOUGH.
Now, save your game, and go up to battle the level 47 Dialga, the pokemon that can greatly help your team to an advantage!

So I'll admit, I was too laxy to write out a whole walkthrough, but you were to lazy to look. For more details, go to this site:
Http://www.gamefaqs.com/portable/ds/file/925601/45138

When you said "the complete walkthrough", if you mean you want a walkthrough of the entire game, use this one:

Http://www.supercheats.com/nintendods/walkthroughs/pokemondiamond-walkthrough05.txt

I used that when I got stuck. It's really helpful.
Imperialist

After you beat team galactic in their HQ go to the top of mount coronet from the hearthome city sside take bibarel since he could learn alot oh HMs you will need rocksmash strength defog and surf take repels in case at the top you will battle to galactic admins then cyrus but you don't get to save after you beat cyrus and battle dialga

I can't tell you the complete walkthrough, but I'll try. After you beat the 7th gym fly to Hearthome city. Go through Mt. Coronet and get to the top of Spear Pillar. Heal your pokemon. Then you battle Commander Mars and Commander Jupiter with your rival. Next your rival heals your pokemon. Then defeat the leader of team Galactic (I can't remember his name) once and for all! Once you beat him you battle Dialga. I hope you didn't use the Master ball you got earlier or it's going to be really hard. If you did I can't help you with that, sorry!

I already answered, but I'm just stating for those who like to gloat, save it for the forums, don't rub it in the faces of those who ask questions. Thank you