Going to assume this is your first time... so if not, you'll probably want to ignore this. LateBloomer's advice is fine, so I won't go over anything he already mentioned, as that'd be pointless.
The mistake I made on my first time bottoming (and his first time topping) was thinking that he could just stick it straight in and everything would be fine. It didn't work out that way
Before he tries to insert himself, you should make an attempt to relax your anal muscles a little bit, with fingering for instance - even if it's only to apply a little lube (also a must for a first time, I'd say) it'll help things along a bit. In general, relaxing yourself (breathe, breathe, breathe) will also help, because if you're too stressed or scared, your muscles will tighten and try to keep him out.
Relax properly, and things will hopefully go a little smoother. Once he's managed to get a bit of himself in, the key is to take things slow for a bit as you adjust, let him push in a little at a time, then wait for a bit until you're ready for him to continue. If you try to force it all in at once you can risk hurting yourself.
And on that note, if you feel pain at any time stop. Simple as. Take break, sit down, and wait a bit before trying again. Maybe with more lube, and another attempt at loosening yourself up with a few fingers (your own or his, some people consider this to be a decent form of foreplay, but it depends on the person, it's probably not for everyone).
Make sure you discuss everything beforehand, so as not to get any wires crossed - and wear a condom! Putting it on for him can be a good form of foreplay, and also helps you ensure that it's on right, so your safety is a little more assured. If you change position at any time, make sure you check that the condom is still attached (and do so every now and again anyway, just to be sure).
Other than that... the rest is really all experimentation. I don't think it's possible to give good "sex tips" because we're all so different, we're all capable of, and enjoy, completely different things. Communication is key, ask him how things feel, tell him how you're feeling - say what you like and what you don't, give him ideas, and work together to make it as enjoyable as possible for both of you.
Hope this helps