Positition effect it a lot! I highly recommend lying on your side with one or both knees tuccked up in front of you, he then lies bhind you on his side (spooning style) and slides in (with LOTS of lube - you can never have too much). Oh and relax!
This is the most comfortable and supportive positioning I have found especially good for beginners. he cun also reach around to the front of you with his hand - he won't be able to fall over this way ;-)
I also recommend using a clit vibe with any anal. experiment and have fun! :-D