Well, I'm just going to get the 'yucky' bits out of the way straight off: If you're worried about cleanliness, DEFINITELY invest in an anal douche or enema. There are many how-tos and reviews just here on Eden to help you through that process! Regardless of use of those devices, take care to make sure your gut isn't acting up and you've recently used the restroom.
To be completely honest, it's pretty much impossible to have perfectly clean and sterile anal sex. As far as stretching goes, there seem to be two camps as far as penetrative anal sex goes: the "just shove it in!" and the "take it slow." I'll definitely throw my hat in for the latter. Especially if it's your first experience with anal sex, just trying to jam it all in there without prior "warming up" can be dangerous and painful. If you're bleeding even a little bit -- stop immediately. That means you're doing it wrong and need to take things slower.
Try having your boyfriend help you out beforehand with his fingers, or perhaps a slender toy (make sure it has an anal-safe base!) You'll certainly need lube, and my personal recommendation for you would be a thick, water-based lubricant. Water-based is great because clean-up is easy, it doesn't stain, and it's totally safe with any kind of toy or condom. If you're unsure about how much lube to use when you're just learning, always err on the side of safety and use more.
As many anal players will tell you: try to RELAX. Being tense can lead to unnecessary pain. Take things slow, and gradually work more fingers in to warm up. If you feel like something is "impossible" to take anally, within reason, chances are you just haven't stretched out enough yet. Trust your partner and make sure to COMMUNICATE with him; tell him what feels good, to change speed, or to go deeper/shallower if need be. Maybe it's a little unromantic at first, but if you enjoy anal sex you'll soon pick up how to enjoy it without having to communicate as much. Don't just take my word for it, though. Many here at Eden will cover bases I'm sure I've forgotten to mention. Best of luck to you!