Yea, I've donated blood.
The test your iron stores using a portable test (quick prick of the finger) and if they are sufficient then they'll let you go ahead. This test is the most painful part of the whole procedure so yea.. the blood taking doesn't hurt very much at all.
Have a nice big healthy breakfast and lunch and you'll be fine. I had toast and cereal for breakfast and for lunch two sandwhiches, an apple, and some juice. Didn't feel like fainting, it was quite a relaxed proccess. But if you do feel like fainting or drowsy/dizzy then they'll stop immediately or put your chair down so the blood flow to your head increases.
They check your weight before you do it so if you're skinny that would mean less weight so they could possibly restrict you from donating.
It doesn't matter if you're vegetarian or not, theyll just be checking your iron stores, weight and some personal medical/health details.
We had a portable van come to our school because we usually get heaps of people donating so it was pretty relaxed and fun. We had competitions to see who could donate blood the fastest lol. The record was a girl who did it in like 3 or 4 minutes or something similar.