Save first of course. There'll be a fight, then save again. Now grab the block and push it over to one of the depressions in the ground. Make Yorda stand on the other one. The door will open. Leave her there and go through.

Go in, up the ladder, up the chain, and out the door. You're standing over the graveyard now. Push the block down to the main level and jump down after it. There'll be yet another fight. So fight and save. Now push this second block over to where Yorda was standing and the door will stay open. Go through.

The Ico Walkthrough

Please swing by my Forum to ask me how to get through if you're stuck somewhere I haven't put up yet. I'll get back to you quickly.