My tattoo artist explained to me several weeks before I got my first one that at its worse, it feels like dragging the tip of a knife over a sunburn. That's actually pretty damn accurate, too.
It's NOT an unbearable pain. It hurts a bit at times, but it's definitely tolerable, and you can just sit and grit your teeth through it. You've selected a very good area for your first ink. The outside of the upper arm is one of the least painful places to get inked, and should be fairly easy to sit through.
But if you can imagine dragging the sharp point of a knife or scissors across your arm...not enough to puncture the skin, just enough to leave that white scrape, you'll be fairly prepared for the sensations ahead.
Also consider the fact that most people who are tattooed usually have several. I personally have 6. 3 of them are quite large (6-10 inches in length and up to 6 inches wide), and one of them even required 14 needles in the tattoo gun at one time to complete (not to mention 4 hours time in one sitting). And that was my second tattoo...I still went for 4 more after that...LOL
As for aftercare, your tattoo artist will fully instruct you on all of that. As NG stated, have neosporin/triple antibiotic ointment/Diaper Doo or something similar on hand for afterward. You will need to gentle cleanse the area a few times a day, and reapply a thin layer of goop over it each time you do. After about 3-5 days, you can let it go without any good, but still keep it clean and try to keep it free from clothing...especially tight clothing...for up to 2 weeks afterward. Ibuprofen works well for reducing pain and swelling.
Good luck! Let us know how it went!
Mimi