I'd say OandA or FXCM Micro(more leverage, though that may be over now), it takes a long time to learn how to trade. I started with $100 when I was 15 at OandA, I ended up losing about 3k in the first 3 years of trading, but have made 20k in the 4 years after that, and now I just got a job as a trader at a hedge fund.
I am the youngest trader there, but I don't think that's a bad thing. IMO if you want to be a trader you have to start trading early. It takes time to master your emotions and really learn how to be a profitable trader. I'd say about 5 years of losing and frustration before you start to make money on a regular basis.
You can either start now, and be ready to go when you are 23 (assuming you're 18 now), or wait 'till you graduate from college at 21 and not be a profitable trader until you're 26.
I worked as a Registered Rep, aka stock broker, aka glorified telemarketer for about 9 months, and I hated it. If not for my trading I would still be doing that, which is a nightmare. If I didn't dedicate myself to being a trader so early I'd be stuck doing something I hate, instead of something I love. Trading professionally is different than trading retail, but the retail experience I have certianly helps.
Long story short, if you know what you want to do when you grow up, start training for it now. Sooner is better. Always.