Irrational and rational proofs

Hey, I was wondering about proving that square roots of numbers are rational or irrational.
I know about proving this for the numbers 1(duh), 2 and all multiples of 2 and 4, but what about 3?
How do i prove that 3(or any other number) is irrational or rational?

That is really awesome, except for one thing, it relies on a theorem. Therefore cannot be considered a proper proof, however it is incredibly easy, now to find a relationship between all rational and irrational proofs to find out more about the world. Oh god....