Not long ago my girlfriend and I started trying anal sex. Let me tell you, I was kind of nervous about tearing/scratching her in there. But if you use a good bit of anal lube you'll be fine. I'm no pro, but we find it best right now if I rub the lube on both my penis and in her rectum area. I can't stress this enough, be relaxed and don't do it doggy, not at first at least. Speaking of relaxation, the position should also be. That's why I wouldn't start off with doggy or you ontop, for too many muscles are being tensed. Spooning or face-to-face is great. You can look into his eyes and he can massage you, reenforcing the serine state of mind. Relaxation and lots of lube...that's essential