Help!

Okay so my boyfriend wants me to go on top. I want to try it but the thing is, I don't really know what to do. Like people say to move forwards and backwards.. But isn't that just like humping him? Moving up and down would actually be like a sex movement but how do you do that? Like if your knees are on both sides of him what do you do with your hands and how do you move your hips up and down without like putting pressure on him and hurting him or whatever?? I don't get it and I don't really want to embarrass myself.