Before you charge ahead and try to get the cock jammed up the butthole, it'll help you both if both of you know a little about the anatomy of the ass.
When you look at your anus... ahhh, not that flexible? Get a mirror. Ok, so when you look at your anus in the mirror, you'll see that beautiful chocolate starfish. Or yours might be another flavor. Human skin tones vary dramatically so expect yours to look a little different to anyone else's unless you've found your anus doppelganger? Wohoo! Anyway, that tinted, puckered area of skin covers the outer anal sphincter.
The skin itself can evoke a positive sexual response if gently stroked with a finger or tongue. If stroking with a finger, adding some saliva or lube is a good way to get a smoother glide, otherwise your finger may irritate rather than stimulate.
Under the skin, as mentioned, is the inner sphincter and the outer anal sphincter. The outer is thicker and surrounds the inner. Both of these muscles are at the entrance to your anal cavity. When you aren't thinking about it, the outer muscle is in a state of partial clench, the inner is always fully clenched.
You have some control over the outer muscle. Clench your butthole and it's that sphincter you are squeezing. That is conscious control - similar to moving your hand. It does what you tell it to do.
But similar to the way your eyes blink without you having to think "blink" every time, the outer sphincter is actually only partially under your control. The same way you blink without thinking when something goes flying toward your eyes, your outer anal sphincter will automatically clench shut when someone touches it from the outside. At least, initially. It can get used to touch and relax.
Your internal sphincter is a bigger problem. Because it is always completely clenched tight, it doesn't want you to have anal sex. With some preparation, you can get this sphincter to cooperate. (We'll talk about preparing for anal a little further on.)
Passing the anal sphincters, you enter the anal canal.
The job of the anal canal is to tell you when you need to poop. Normally, poo will stay further up in your intestines until it's ready to be pushed out. (Hey, can't talk about anal sex without mentioning poo.) When it drops down into your anal canal, you can feel it. You get that feeling and you know it's time to go. So when you get a cock or dildo in there, initially you are going to feel like you need to go. That's natural, normal and though quite disconcerting, it passes. That's one really good reason why you need to get comfortable with how things feel up your butt by yourself.
Compared to a woman's vagina, a butthole is much less suited to penetration. It doesn't self-lubricate, it's made of less sturdy stuff, and as many people who don't like the idea of anal sex will quip: "It's an exit only hole." That is what it was designed for but it doesn't mean you can't hack it for a positive perverted pleasure. You do need to take care though.
Having a large crap, hard stools that are the result of constipation, even a bad case of diarrhea can cause a tear on the inside of your anal canal and result in a little bit of blood. So you should know that for your delicate dixie-hole, even gentle anal sex can be as traumatic as any of the previously mentioned ailments.