lube is a must, I would suggest a silicone based lube (not to be used with silicone toys however) since it requires less re-applying, however test out a variety of lubes and see which works best for you. The anus has 2 sphincter muscles, one you can't voluntarily control, and one you cannot. Using a well lubed finger or toy you can slowly coax the second muscle to relax and open, this should not be rushed since the more relaxed this muscle the more enjoyable, and pain free you will be. If you experience any pain stop what you're doing, take is slow, you don't want to be feeling pain.
Good luck! Understand that anal sex isn't for everyone so do not feel pressured into doing it if you yourself don't want to, you should be getting pleasure from it as well, not just the top.