1. How and where can I get a Giratina and Cresselia?

1. First, you need to beat the E4 and obtain the National Dex.
Giratina- found in Turnback Cave along Route 214. The cave is totally random, so just go in random rooms and you'll eventually run into the room where Giratina is. However, if you go in the room you just came in from, you'll start from the beginning. So be careful.
Cresselia- talk to Sailor Eldrich in Canalave. He'll take you to Fullmoon Island. There, you'll encounter Cresselia and she'll run off. However, you came here to take the Lunar Wing, which Cresselia left behind. Go back to Sailor Eldrich, and use the Marking Map to track down Cresselia.

2. The storyline is over once you've encountered all the legendaries, but the game is definitely not over. You can try catching all the Pokemon in the National Dex, minus the Event Pokemon, Manaphy, and Phione. Unless you cheat... You can cheat to get all 493 pokemon. You can also win 100 times in a row in the Battle Tower, obtain the Master Ribbon in all 5 categories... Do some stuff underground, play Capture The Flag with a friend... There's a lot to do in the game. When you beat the game 100%, tell me and list off what you've accomplished. I bet I'll find something you haven't done.

Pretty much, it is. Now all you have to do (if you haven't) is
1. Beat the battle tower
2. Get master rank in all the contests
3. Get a complete pokedex

You must beat the elite four first
Giratina

1. Pull out you town map
2. Look for a place called spring path
3. Go through the cave until you see him
4. Finally CATCH HIM!

Cresselia

1. Go to Canalave City
2. Go to the house nearest to the person who took you to iron island
3. Enter the house
4. Talk to the sleeping kid
5. Talk to the mom
6. Talk to the sailor outside
7. Go fullmoon island
8. Walk around until you see cresselia
9. Pull out your pokeradar
10. Track it down and CATCH IT

The story is not over once you catch Heatran you still have to beat all the levels in the Battle Tower.

1. Giratina is at the lake south of Veilstone (not Lake Valor) and Cresselia is on Fullmoon Island (sail from Canalave).

2.Everything after the Elite 4 are basically side-stories, primarily separate ones, meaning that you've finished Heatran, but there still can be other branches of the story to play through (like Giratina and Cresselia).

1: Giritina is in the Knockback Cave indide Jagged Path. Cresellia runs.

2: Not a whole lot. You can capture all the pokemon.

1. Giratina is on springpath in turnabout cave? I think thats the name (I,ve already caught it) it's quite a confuesing cave some people have said but I guess I got lucky. Check walkthroughs. 2. No you must finish your training card by beating the battle tower lots of times (cant remember how many), beat all contests at maximum diffuculty, get every Pokemon, and 1 other thing. Anyways the more stars, the more popular you are. hope I helped