How do you castle?

I kinda know how to do it but in this position its kinda already castled (except blocking the rook in), i did not move the king, so how do i castle? (Im white), thanks.

You need to move the d1 rook because after you castle "a-side" your king will be on c1 and the a1 rook will land on d1.

oh okay thanks alot!