I barely remember my first time. I wasn't raised to think sex was a special thing. It was just a normal and natural thing and I didn't pay much attention to it. I was 15 the first time. I can't count the number of people I've slept with. I mean, it's under 50, but I would have to stop and think about it. It hasn't mattered to any of the guys, at all. I genuinely liked all of them (at least at the time) and sex was an expression of that. Nobody has treated me like a whore. I nearly married someone I fucked on the second date, and am currently engaged to someone I banged on the third date. Some relationships haven't lasted, but some have, and sex was anything but the deciding factor.
The best thing you can do is relax and have a good time. If this guy has been with you for 2 years, then he's not planning to bail on you tomorrow. Nearly every STD is curable, and the ones that aren't are highly treatable. Your likelihood of getting AIDS is pretty low, because managing it requires a battery of medications and I'm guessing you would notice if this guy you dated for two years had a chronic health problem and a bunch of meds. Herpes would be annoying, but most people only have one outbreak and there's medication to keep it in check. HPV usually goes away on its own after a couple years, but there's also a vaccine for it if you're super worried. There is the risk of cancer, but that's what pap smears are for. Cervical cancer is a very slow growing cancer and is pretty easy to catch early as long as you get your checkups.
As for pregnancy, use a condom and a backup method. If you don't know how to use a condom, ask your doctor. Practice on a banana. Get some birth control pills and take them at the same time every day. Get a nuvaring if you are forgetful. Your odds of getting pregnant will be practically zero. Nothing is perfect, but there's a risk with everything you do. Now is a great time to have the 'what if' talk, but everything is probably going to be fine.
If everything isn't fine, come back and we'll help you figure it out.