Nice to know my internal size will be unaffected then, and I plan to use a condom so i dont want his cum inside my bowels or anything. I don't want any sexual diseases.
Do I have to douche myself, or can I just clean myself normally?
If you do not douche, you run the risk of having shit appear in the middle of the action; take it from someone who has been fucking ass for probably longer than you've been alive (34 years now), it's a complete buzz-kill.
Douche with water, not an enema. You can buy a standard Fleet-type enema, but empty the bottle and refill it with warm water, In a pinch, an empty drinking-water bottle would work, too.
Before you jump into a sexual thing with anybody, you should learn some things about your body, and how it reacts to different sensations, first. That means that you need to do some exploration with your fingers and, perhaps, a modest-sized toy. Get some lube and go slow.
Start by lubing the outer surface of your anus, then begin on the sphincter. You'll probably notice that it grips pretty tightly: now imagine having a dick pushing through instead of just a finger. You will have to learn to relax your sphincter if you expect to get penetrated and get any pleasure out of it.
I'd recommend several attempts at experimentation solo before bringing anyone in (pun intended).
Good luck!