There's a lot of advice available online that will probably be way better than mine but here goes: -
1.) if you CAN, use a toy or such on yourself to 'practice' sometime before the big event, this will make life easier for you and you'll know what to expect.
2.) To clean your ass you can buy a douche (a squeezy bottle for your butt) or give yourself a good wash back there in the shower after the bathroom. Standard hygiene. On a gross note I find the 75cl plastic Evian bottles make a good economical douche. If you decide to do it use only
lukewarm water and NO soap... do this a few hours prior. It is not strictly necessary but if you are really worried about hygiene it may help you lose yourself in the moment a little more
3.) sorry to break it to you - but it will hurt >.> at first...then it'll feel amazing. Then you'll start seeing what all the fuss is about. Then you'll be finished and want more ^_^
4.) I would really recommend the 'cowboy' position to start with - him laying on his back, you straddling him and sitting down on his cock. With this you will have much more control and you can paw off easily when you feel ready. After you start feeling okay with it you can try others, there's a lot of positions.
5.) use lots of lube. I'll say it again - use lots of lube
like Fenris says, being relaxed and eager is key to enjoying it, so just take it easy and have fun.