It's not painful other than a pinch (twice, see the iron test).
You do not have to know your blood type; they will let you know what it is by mail. I determined my own long ago - now that was fun!
I won't lie: the needle used to draw the blood is a little larger gauge than needles that may have been used to draw your blood in the past, but it is bearable. Plus, you get all kinds of treats after you donate!
Plan on 25-40 minutes for entry to exit as you'll want to sit and eat/drink something for a few minutes before you go. You may feel lightheaded after donating but that is normal. I haven't seen anyone faint but it does happen occasionally. There is someone there to walk you from the donation area to the feasting area so you don't really have to worry about fainting.
It's a great community service that benefits many people. It's also addictive. After you do it once, you start looking forward to your next eligible donation date. You may give blood every 56 days.
http://www.redcross.org/donate/give/
[This message has been edited by Spiff (edited 10-10-2002).]