This is yet another reason why you need a sex partner you can trust for any sort of anal sex. Lubricants and condoms have one other advantage. They prevent feces from sticking to the penis. Most screwers aren't bothered by a small amount of feces, but it can be embarrassing for a novice screwee. Usually no extreme measures are needed to clean the rectum beforehand, just a recent bowel movement.
Some people douche.
This can backfire since water left behind can be more embarrassing than the tiny flecks of feces the douching was intended to remove. Take your time. Start with one finger, working lubricant into the anus. Unless you have an unusually slim penis, work up to two fingers before trying to insert the penis. When you use the penis, move in and out just a fraction of an inch, gradually working your way in.