Ditto to all the above on preparation. You really do need to be building up your tolerance and stretching things out to help you relax. As for damage - I sort of have to disagree with Oz on this one. Some anal sphincters are a lot smaller (or at least tighter) than the average penis and need a smaller insert to get started otherwise they will tear (especially if not lubricated sufficiently).

Which reminds me of something no one has mentioned yet - lubrication. Lots of it. If he is going into you for the first time with his penis then you will need a lot of this. It will certainly make things more tolerable (but not entirely so). There is a tendancy in some fantasy BDSM porn for subs who 'take it without lube' but I have never known anyone who could do this in real life - at least not without a lot of prior training. Someone who hadn't actually done this before but had just read about it may think this is possible. Not saying that he is someone like this (I really hope not, for your sake) but it is something you need to be aware of. Chances are, he knows this and will have the lube ready. If not, you know about it now and can do something about it.

Another concern I have is safety. He should be wearing protection and that protection should be a fairly robust condom. I say this realising that he may have already thought about that and therefore being somewhat patronising but it is an important consideration in any sexual encounter - whether it be vaginal or anal or oral. Sexually transmitted diseases can be transmitted by any sexual contact and you should not be having unprotected sex with anyone, no matter how much you trust them or they claim to be 'clean', until such time as you have both been checked by a clinic.

Hope all this helps. Apologies if I am stating the obvious in some cases...