My favorite body mod...
Outside of the arm hurt hardly at all for me, which is the general consensus from what I've heard. Inside hurts a lot for some people, just pinched a little bit for me (I also had a great artist who has a fairly gentle touch<plug>Carlos at Rising Dragon in the Village, NYC</plug>) However, have you thought about getting it done somewhere else like on your back or leg? It might be a bit more unique, and it will certainly be easier to hide (always ALWAYS important!). If you're not sure, DO NOT GET IT DONE! Ask a friend to do something in henna, for instance, and try that out for a few weeks, do your research, don't be impulsive. Coverups are expensive and require a skilled artist; removal is also costly and painful, and not perfect.
The tattoo will take about a month, give or take a few weeks to heal (beyond scabbing and tenderness), but I'd be careful about it as long as you can. Use vitaminA&D cream or lotion *only* -- no petroleum jelly, no antibacterials (unless you are super-prone to infections) and do NOT bandage it. You should also lightly wash twice a day with a mild soap and pat, not rub, it dry; also, avoid long showers/soaking the tattoo. As several people have mentioned, don't scratch, itch, or pick at the scabs. Most aftercare that I've heard also recommends avoiding alcohol and other drugs before and after the tattoo.
I did the evil and picked a tattoo parlor nearly out of the air (one with a good reputation, of course), but there are some definite things to watch out for: ask to see photos of the artist's previous work so you can get an idea of what their strong points are and if you like their style. Occasionally, you'll also be able to talk to people who've had tattoos done by the artist. Absolutely check that fresh needles are used for each customer and that guns are *autoclaved* between every customer. Reputable shops will post or have available bacteria tests (that are done regularly) that show how safe the autoclave is. Definitely make sure you're comfortable with the shop and that it's clean -- you don't have to go to a hole-in-the-wall.
And the needle gun sounds scarier than it really is
~Ladi