From a woman who knows, if she is asking you she will already be fairly relaxed so that will make it easier. Use lube and start with your finger, before you are ready to penetrate, apply more lube and also on your penis. Go very very slowly an inch at a time and give some time for her body to adust to you being there before you thrust. I think best position is lying in spoons, I am most relaxed in this position.
Use plenty of k-y,start with your finger then move up to your **** ,penetrate very slowly to not cause her any discomfort, and let her control it- the best way is with her on top.