how to eliminate the pain?
Practice with a dildo in your own time to get used to your body and the sensation of accepting a dick into your ass.
Make sure he prepares you properly before fucking, with rimming, perhaps a bit of fingering if you like it, make sure he uses plenty of lube (on your ass and his dick) and goes slowly to start.
Something I read recently which makes sense is get him to press against your asshole very gently with his dick and actively tense your asshole, after a short time your asshole will naturally relax and penetration should be more comfortable.
A drink or two beforehand could also help relax you.
Try different positions - everyone is different and some positions might be more awkward and uncomfortable than others for you. A lot of guys like to sit on a dick to start with as then they can control the depth and speed of penetration.