So I want to get fucked in the ass for the first time. How do

what do you mean? the actual fucking or getting to know someone who would do it?

easiest is proabably with a guy, who isn't huge. my fiance was also my first time and he has a smallish dick. it didn't hurt too much, but it's still long enough to be pleasuring.
lube up, like a lot of lube. if it helps get some poppers. it helps my fiance if he didn't bottom for a while (because I'm rather well equiped), does nothing for me though.

make your partner go slow. honestly, you'd best do it with your bf or atleast someone you know and can trust. anal sex is a lot about trust for a lot of guys, especially if they're unexperienced.

first time is mostly about the right mindset, you have to be the type to do it with some random guy.

as for position: I tried riding my bf. despite the fact, that this is hardly enjoyable if he has a smallish dick, it also didn't really work because I was too tight and couldn't loosen up at all (stress). so I let him do it all on his own, what requires trust.