What should I know before donating blood/plasma for the first time?

If you're just donating plasma, they will put the red blood cells back in you. It will be cold and mildly disconcerting. Whether you are donating just plasma or blood, you should not drive home. You should rest and drink plenty of liquids like juice and gatorade. I'm not sure about donating blood, bit most of the tine when blood is drawn, they use a needle to poke a hole before taking the needle out and putting a thin tube in your arm instead. It's a little scary, but bearable. Good luck!