I don’t know how to be on top

This is going to sound ridiculous. I’m a grown ass woman with two children, in a long term relationship, and I’ve never been the one on top during sex. I’m scared to death to. My partner is always asking for it. He wants me to ride him, but I refuse because I’m embarrassed because I don’t know how!! Literally I don’t know what I’m supposed to do/how to move my hips. I’ve also gained a fair amount of weight after two kids and I’m uncomfortable with sex in any capacity now. I just don’t want to make a fool of myself, but I’d actually like to try this position. It seems so basic but I can’t do it! I don’t know what to do other than to try and avoid it for the rest of my life.