The blood taken is not that much, it's less than a pint I believe. I'll be honest, the needle is big looking, but it's not that bad. A quick pinch and you're done. Some people feel a little itchy where the needle is in but it's nothing that will drive you crazy, besides the nurses make sure they have steady hands!
You'll be alright! promise
Just think about the fact that you're doing an awesome thing for someone who needs that blood.