You know that F(x) = P(X\le y), right? So F(y) = P(Y\le y) = P(\sqrt{X}\le y). Use F to express that probability.
If you are still stuck, please show some sort of work so we can see where you are stuck -- if you can do no work on the problem, show us an example you found in your textbook or notes that is similar to this, since that's the first step in solving a new type of problem.