Lucario..

Use a fire type Pokemon it has a big effect on lucario because his type is steel.

You need a Pokemon that knows a good ground move preferable EarthQuake or one that knows a strong fight move or better yet Drain Punch. Or just level up your Pokemon to a level higher than Lucario's Level.

Someone gives you the Pokeradar. You can consult the Walkthrough page under the Pokemon Diamond forum.

Lucario, being a fighting and steel type Pokemon, has a variety of weaknesses.

A flying type Pokemon would be your best bet, as it could kill her whole entire team. However, as a back up for that flying type, you might want to use a ground type against Lucario.

An easy flying Pokemon would be Starly. If you are able to level him up to level 34 and evolve him into Staraptor, and you know Aerial Ace, this battle should be easy for you. Use Staraptor against all 3.

A good ground Pokemon at this point in the game might actually be Geodude, Graveller, or, if possible, Onyx. Make sure they know a good ground move, like Magnitude. If possible, Earthquake is prefered. Use the ground Pokemon as a backup when against Lucario.

All you need to do is find either a flying type Pokemon (may I suggest a lv. 30 staravia or prinplup/emporeon) or a psychic such as abra. Drill peck for empoleon works extreme wonders and wing attack with staravia might work...

I used my staravia to win and also you get after you beat the elite four

First off, the poke'radar is gotten AFTER you get the National Dex. Secondly anybody with a high speed and a ground move will work.

I'm sorry, I forgot to answer your second question >_> And seeing as how no one else answered it to an extent, I'll answer it.

Unfortunately, you're nowhere near the part in the game when you can obtain the Poke Radar. You're on the third gym (or, may or may not be on the fourth or fifth gym, at this point). What happens is that you need to obtain the National Dex, first. This is done by seeing all 150 Pokemon in the Sinnoh Dex, which is made easier AFTER defeating the Elite Four. And you can only battle the Elite Four AFTER you've obtained all eight badges. You still have to obtain 6 (or less) badges, plus the storyline along the way.

For reference, this is a guide by Merriland to help you obtain the National Dex:
Http://www.gamefaqs.com/portable/ds/file/925601/45138
Once you're there, click "CTRL" + "F" and then type in "s8.6." This will bring you to the locations of the Pokemon found in the Sinnoh Dex. Once you've done that, go to Sandgem Town and go in Rowan's lab. Oak will come up and give you the National Dex. With this, go to the end of Route 221, and go in the building you see, there. Oak will appear and tell you that this is Pal Park, and he'll give you the prize you've wanted all along: The Poke Radar!

Get a flying type Pokemon to beat all the Pokemon (except lucario) for lucario use a fire type pokemon.

To get a pokeradar, see all 150 Pokemon and talk to proffesor oak at the Pokemon park.

I used a Monferno.