proof of infinite n's s.t. sqrt(n) = irrational

Hey; I need help with the following problem:

"Prove that there are infinitely many positive integers 'n' such that sqrt(n) is irrational."

I think I'm supposed to do this with a proof by contradiction, but I'm even struggling with that distinction... which part do I negate? the infinite part or the irrational part? :(

I'm really struggling with my proofs involving irrationality... if anyone could give general tips concerning irrationals in proofs, I would also greatly appreciate it.

Thanks,

Martin