But I don't have any games for it.  So tell me, what should I get?

First on my list:

Phantom Hourglass
Pokemon Diamond

Later on:

Professor Layton
Ace Attorney
The World Ends With You
New Super Mario Bros.

What else should I put on my list?

Congratz man you should get New Super Mario Bros. I beat that game 100% I really like it.I will also reccomend you Yoshi's island and Mk DS.

Ninja Gaiden: Dragon Sword is a must have let me tell you!!

Elite Beat Agents for sure.

Umm the FF advance games for GBA are brilliant.

Mario Kart is fantastic.

I really enjoyed the 'Feel the Magic' minigame collection for the price I got it at (around $10 US equivalent?).

Metroid Prime Hunters isn't worth it.

The advance wars games are worthwhile as well.

Mario and Luigi: good RPG that involves a lot of button smashing

elite beat agents: Good Music game, Addictive, challenging!

Animal Crossing: Good Simulator game, spend about 100 hours with it myself

and congratz :)

-New Super Mario

-If you like classic platformers with a lot of challengue, pick Yoshi's Island DS. Very good game, and if you want to complete every secret level, you are going to experiencie a great challengue.

-Elite Beat Agents is definitely a must buy.

-Both Nanostray are very good, with amazing graphics.

-But, don't forget, now you have in your hands the system with the most third party and RPG's support. Anything you want: Atlus, Square Enix. Pick any Final Fantasy, The World Ends With You, a Mana series game.

-Phantom Hourglass is very good too: I promise you, you will love Captain Linebeck.

-Mario Kart is awesome too.

-Mario & Luigi Partners in Time is very good if you like RPG's. Very funny game and very well writen.

Tetris DS
Elite Beat Agents
Final Fantasy VI Advance

Also, Geometry Wars Galaxies isn't bad for some quick fun.

Final Fantasy III

No Advance Wars love? The Ninja Gaiden DS demo was pretty good - try it out if you have a Wii.

Did I mention that you simply must be Elite Beat Agents.

Seriously, out of all my DS and Wii games that one takes up at least 75% of my play time. Its simply amazing.