I guess the safe sex moniker is what got to me...
In addition to what ausmaxx said, foreplay will actually cause your anal muscles to relax as well, so don't just strip and go right into the act, take some time to kiss, stroke, etc., whatever you want to get the ball rolling. That will help.
Loosening yourself up with fingers is a great way to start, either yours or his, either way (make sure whoever's doing it has their nails trimmed and filed! Don't want to cut up your insides with long and/or jagged fingernails). Start with one, work your way up to three, and don't rush it too much, your muscles can only acclimate to being stretched so quickly.
Lots of people say that if you're an inexperienced bottom, it's best to sit on top of your partner so you get full control. I disagree with this statement, having been a top with someone who was not loose, it can be very painful to be sat on when the person sitting is tight. Doggy style will give you some room for movement while also being more comfortable for him. The unfortunate side effect is that doggy style allows for a deeper penetration than laying on your back, but you have practically no control when you're on your back, so I think it balances out.