What 89blkcivic said.
All in all, it takes about an hour from beginning to end. They take about 600mL of blood from you. Your body contains about 6L of blood so it's not a huge amount by any means.
When you get up off the seat, try to get up slowly Sit yourself on the side of the chair and make sure you feel OK before you actually stand on your feet.
It actually isn't that bad, but it's good to take some precautions if you've never donated before and are unsure of how your body will handle it.