Honestly, I am officially the worst person on the planet with needles, and the worst bit about the blood test I had the other week was peeling the plaster off afterwards cause it was really sticky...
But seriously, if you're nervous, tell the nurse/doctor who's doing it, and they'll calm you down and take you through it step by step. My nurse made a joke about me fainting which really put me at ease. It really didn't hurt that much, and my vein stopped giving halfway through so I had to tense my arm, which was a little painful, but not enough to make my eyes water or anything.
My arm ached for a day or so afterwards and I had a massive bruise, but it was kinda cool, lol!