I used to give plasma before I got a real job to get spending money in college.
Plan on either getting to the center really early or spending half an hour to an hour waiting in line. Plasma centers are pretty popular. Figure in at least an hour to actually give the plasma.
It doesn't really hurt, unless you get a bad technician or you have small veins. Sticking a moderately sized needle in your arm multiple times isn't very comfortable.
The saline solution they pump back into your body is cold, relative to your body temperature, so there's also the chilly sensation of feeling ice in your veins.
Don't forget you'll need to get a physical the first time you give plasma. Figure in another hour of waiting in line, urine sampling, and actually getting the physical. Plus most centers only have their doctor come in on certain days, so call up there before you go.