i seriously doubt that you are serious, but if you are, this is what you need to do...
1 -- get one of those shower heads with a massage head attached to a hose.
2 -- unscrew the head off so you just get a single stream of water.
3 -- run that stream of water up your ass for a few seconds, then push it out.
4 -- continue doing this until the water coming out of your ass is completely clean - then do it some more!
5 -- now that you know your ass is clean you are more likely to relax and not feel self-consciouss.
6 -- don't use "astroglide", get a bottle of "wet" and put a bunch of it on his dick and in your ass.
7 -- start in missionary position and have him SLOWLY go inside. if it takes ten minutes to go all the way in, let it take ten minutes. the first part's the hard part, so take your time.
8 -- at first it will burn a little and maybe even seriously hurt -- DON'T GIVE UP!! the fun's just about to start!
9 -- have him go in and out of you very slowly for a while until your ass relaxes completely and start to really enjoy it.
10- now have him hammer-fuck the shit out of you while he calls you a dirty bitch until you have a screaming orgasm.
- you will be addicted -
have fun.