There are many threads on this topic.....with man good opinions and thoughts.
Try using different butt plugs and dildos in your ass now, "training" your ass and letting it get used to being penetrated. Start small and slowly go bigger. Over time, your ass will get used to it and that initial penetration becomes less and less painful.
When you start your love making take your time and try to relax. Lots of foreplay, so you're good and turned on...let him gently finger your hole or rim you....get that ass nice and relaxed. Use lots of lube---no matter the position---and let him slowly ease into you. If you have a type of "injector" try to insert some lube into your ass. Relax and breath easy.....
Use whatever position feels best and most comfortable for you....on top, doggie, on your back or on your stomach, with one leg up.......use the one that allows you to be relaxed......if you are relaxed, that's "half the battle".....And yes, a top lover who is considerate and knows you are a "newbie" is important, knowing he will take his time and let you dictate how fast and how deep he goes with his initial penetration and subsequent thrusting...
Once in, have him pause for a moment or two, and let your ass get used to his size inside of you.......try to stay relaxed and keep breathing......when the initial discomfort subsides, you can begin moving....both of you, until you can move together, hopefully in unison, with both of you enjoying the physical and emotional pleasure of anal love-making....
The joy and pleasure of being penetrated is wonderful, and so worth the initial fear and uncertainty......and the orgasms you will feel and experience as a bottom.......OMG.....like no other orgasm....simply the best....