hello i am a newbie and i want to play in high level tournaments and i would like to know what set should i buy to start building my deck

first I would suggest buying some starter decks and a box of a set. It depends on what your playing as seeing as you probably wont be qualified for ATL you can start w/ anything you want.
80 Card starters are
X-Men vs Brotherhood (Marvel Origins)
Batman vs Joker(DC Origins)
Spider-Man vs Doc Ock(Web of Spiderman)

52 card starters-
Fantastic Four vs Doom
Batman vs League of Asassians

Your best bet: Pick up the X-men starter, and when you can, buy a box of the new X-men cards when they come out. This will be your best bet for being competitive. Trying to put together much else is going to involve a lot of rares from many different sets, and that can be a nightmare! The other good points to remember are: A: X-Men will be the new set...even if you pull a bunch of stuff that YOU can't use, it is NEW so someone out there will probably be willing to trade from their collection. This will help to get you access to the older cards that you don't have. B: Your started and booster packs will correspond 100%! Both the X-men and Brotherhood are headliner teams in the new set, and the starter utilizes both teams! C: You'll want the newest cards to be able to play in all tournament formats. You can use new stuff in all of them (Golden Age, Silver Age, Modern...) but old cards cannot be used in all of them (Golden, and maybe Silver only).

Needless to say, you picked a great time to come aboard!

What box should i buy after the starter box

I recommend a box or 2 of Avengers, its fairly new and competative.

Only problem with buying Avengers boxes is that you won't get cards that mesh with your starter.

Buy a box or two of the new X-men set (comes out next menth) and you'll be fine.

As a general rule for ALL TCGs, if you buy the newest set its alot easier to trade for what you need from previous sets.

Yeah. buy
2x X-Men vs Brotherhood Starter deck and a couple boxes of the new x-Men set. Will be out in about 3 weeks.

Personally, I think you should wait till X-Men is out, see how it is and then purchase.