Castling

Hi someone could help me, some times I try to castling but because my rook would be under attack by a opponent piece I could not move. Is that right?

I mean  the King can not be in check or, of course, became in check, but the Rook?

Thanks!

If the rook is under attack afterwards then the King would have had to pass through check.

See castling rules here  http://www.chess.com/learn-how-to-play-chess.html#howtomove

"the king may not be in check or pass through check"

The rook can be under attack or pass through an attacked square to castle as long as the king does not.

It may be that your king is passing through an attacked square, if this is true it makes casteling illegal.

If your rook has made any move, you can not castle to that side anymore.  If your king has made any move it can not castle at all.