By now you probably have already made it through your first donation! Thank you for saving a life. I kind of lose track of different threads (you will also get forgetful when you get old
I am again amazed you can do this under 18 w/o parental permission. That's just for the legal issues. For a medical reason, we use a rough estimate in terms of size and age and gender to try to make sure a) we don't take too much of your blood and b) we have a fairly uniform amount of blood in each unit.
It involves inserting a needle into your arm, so there is a momentary 'pin prick' feeling, like when you have a blood test done. Until my military travels put an end to it, I was a very frequent donor since my college days, and it never hurt me, nor did it seem to bother anyone that I saw donate. I doesn't take very long at all to fill the bag, especially a young healthy athlete like you, you will be out of there in no time, even allowing for all the juice and cookies the red cross ladies will push at you.
Nothing special you have to do to get ready, they will ask you some questions, check your blood pressure, do a test to be sure you have enough iron in your blood, and you are ready. The Red Cross personnel will clean the area that they will be using (bend of your elbow, either arm). You should just eat as you normally do, but afterwards drink plenty of fluids and take it easy that day. Your body will quickly repenish the red blood cells you lost. One word of advice, however: I have found this past year that frequent donors (five+ times/year) that some of our young troops are, some are developing anemia. I think this is in part due to a not-quite-what-your-Mom-would-feed-you diet they follow. Eat plenty of iron-rich foods for the month after you donate. If you are going to donate regularly, you may want to discuss how often with your physician, or the need for a vitamin with iron.
Again, thank you for being a hero. Blood donations save lives; for all the brave talk that goes on around here, giving blood is a truely brave and selfless thing to do.
I hope Santa rewards you!