How do you win as black?


Short answer: White has to make a mistake.
Long answer: You need to study openings. Start out playing only 1.e4 e5 and 1.d4 d5 with Black and learn the ins and outs of the classical openings. (Open Game, Queen's Gambit Declined)

on a different forum u said u cant beat lvl 3 computer...
If he's being checkmated early in the game, then yes, he probably needs to work on learning basic opening moves as well as the opening principals.
Even though opening study can be a pitfall for new players, you do have to spend enough time with it to learn the basics.
on a different forum u said u cant beat lvl 3 computer...
In the other topic he said he loses sometimes to it.

Well, that's your opinion. I don't suppose you have any basis for saying that, besides parroting what you've heard others told.
Clearly he doesn't have to know multiple variations or what lines are popular at GM level right now, but he has to know *something* about the openings that he plays. It will help with learning about pawn structures and planning as well.
Yeah, basically that. I don't mean he should memorize many opening lines and I don't think GCB is saying that either. But it's just general chess knowledge to have seen a few example games from masters in the early 1900s. Knowing the first few moves of a mainline Ruy, Queen's Gambit, Scotch, Slav, etc is just general knowledge. 5 moves or less is what I'm saying.
Of course most important is opening principals, knowing tactical motifs, and diligent calculation of forcing lines on every move.

How do you win as black?
Just play better than your opponent.
How to play better than your opponent? Be better prepared.
How to be better prepared? Study chess more than your opponent.
How to study chess?
There are lots of ways. One way is THE KING OF PATZERS CHESS ANALYSIS, INC:
Other way is learning the marvelous BRAIN FRY ATTACK:
P

To win as Black, just keep playing the best moves you possibly can play and if your opponent makes enough mistakes, you may win.