As a confirmed bottom, I can say LUBE, LUBE, LUBE and relax! Don't go rushing to sit on a baseball bat! Start small and slowly work your way up! I have 3 plugs that get progressively larger. If it has been awhile since I have taken a cock I will use those to loosen up along with ALOT of finger play! If find it helps to "bear down" like I am trying to shit when I feel the tip of the tops cock at my hole. Believe it or not when you do that you are relaxing the muscle that keeps it closed making access easier. I have one top that I play with on a regular basis who loves to eat ass and he works my hole over with his tounge and fingers for at least an hour before he can't stand listening to me beg for him to fuck me and slpis right in. He is the only top that I have been with that I don't need lube with as there is plenty of my own natural juices and his spit flowing! I enjoy using poppers but they aren't always needed. I don't drink or use drugs at all either.
Just remember to relax, go slow, and use LOTS of lube! Have fun with it!