Make sure you eat well before and get loads of iron (apparently loads of slim or young women aren't eligible because their iron levels are too low on the day). To be sure I'd start taking an iron supplement now too. The process itself is fine, doesn't take too long (depends how busy it is! But if you have an appointment it won't be too bad). If you have one arm with more visible veins than the other let them know because when I went the blood had to be taken from the side the machine happened to be set up on, and I ended up having to move beds.
Don't worry too much about dizziness afterwards, they don't let you get up for about 10 minutes and even then you chill out with a biscuit and cup of tea. I just felt a bit lightheaded, same as if I was hungry or tired. I was fine after an hour.
Bear in mind if you do a lot of sport that the decrease in red blood cells can affect your performance for a few weeks, but this isn't really anything most people have to worry about!