It's not usually painful at all! Just a little prick as the needle goes in. I sometimes get bruises afterwards, but it's not painful! You'll roll a tube in your hand during the process, to keep the circulation going...you might feel a bit tingly or numb in the hand while it's happening, but afterwards that goes away immediately. It only takes about 10-15 minutes, my center has a TV on, so the time flies! When you're finished, there's a little commisary area with snacks. You'll sit there for 15 minutes or so, drinking juice or water, and eating raisins, sweets, or popcorn (for free!). Then you're good to go!
They take great care for sterilization...when you give blood for tests, does your mom think you'll get ill?
Make sure you're hydrated...once I went in after a long, hot day of continuous iced tea drinking...my blood clotted during the process, so it was a waste! Oh, they'll also check your iron level first, to make sure you have enough. Try to eat oatmeal, or something rich in iron, the day you go.
I cannot say enough about blood donation. To think that something so simple, which takes so little time, could actually save someone...it's a great feeling!
I forgot...there is a weight limit, somewhere around 95 pounds or so.