Pro Gamers aren't just 'very good'. Some of them actually use Pro-gaming as a living, entering contests and getting cash prizes!
If you need to ask for tips on how to play Halo 3 on a site like this then you probably aren't good enough for MLG.
Anyway, you won't get a 'contract' with MLG, they don't pay you a salary to play video games. They run gaming competitions and the best players get paid prizes.
I had a look at you're Bnet profiles. You have a losing streak of 10 games in Team Doubles at the moment which was preceded by only 7 wins. A look through the other games tells me you are not consistant, there's no real pattern between you're wins and loses, I might as well have looked at randomly selected 1s and 2s.
I'm sure that compared to the people you're put up against and with in matchmaking, you are sometimes a very good player and compared to the pro gamers you're not.
Sorry to tell you this, but you're not going to be an MLG pro and time soon. But still, it won't hurt to enter competitions with other sites. Search for Halo 3 Tournaments on Google, see how you do in them and build your way up to the major leagues.
