The "prep" once you arrive at the facility will be some initial paperwork, verification of your ID, taking care of the insurance co-pay, then you will actually go back to the waiting area. Once there, you will change into the gown, put all of your clothes into a plastic bag which will be stored under the bed/gurney.
Various people will come to talk to you. You will see one nurse most of the time, and she will take your blood pressure, temperature, pulse and will put in the IV line. I have had multiple colonoscopies and some other outpatient surgeries and I have always had an IV inserted. They need to do that in case there is some emergency that requires an IV injection of something (That rarely ever happens, but good medical practice says to be prepared.)
At some point, your parent(s) will be allowed to come back to your cubicle to sit with you for the rest of the prep period.
Every time a new person comes in to see you, they will probably ask for your name and birthdate to make sure that you are the patient they think you are. The anesthesiologist should come in to see you and ask about allergies or any other medical issues. He/She may tell you about the anesthetic drugs they will use - you can ALWAYS ask them what they will use. A mix of propofol, fentanyl and one other drug I cannot recall are commonly used.
There will be consent forms for you to sign as well. The doctor doing the procedure will also probably stop by for a moment, but that does not always happen.
When the room is ready, you will be wheeled into that room and your identity will again be verified. When everyone is ready, you will get the dose of anesthetic drug(s). I have never been asked to count backwards from 100 or any other stuff that you may see on TV. Not saying that that won't happen, but I think it is mostly a made up gimmick for TV.
Once you have the sedation going, you will roll on to your left side and then you will probably not remember anything else. There may be times when you are aware of the room and perhaps of being rolled to your other side, otherwise, you will simply become aware of things again as you are being rolled back to the recovery area.
The recovery nurse will take your blood pressure and ask a few questions. You will rest in the bed for a while. The doctor should come in to see you to give you the results of the procedure. Your parents should be there for that, since you will not likely recall anything about that conversation. The nurse will give you all the post-op instructions you need to follow (like don't operate any heavy machinery for at least 24 hours!) At some point everyone else will be shooed out of the cubicle so you can get dressed again and be seated in a chair for a while until they discharge you.
You will probably be very sleepy for the rest of the day and should just lie down when you get home and take it easy.
All in all, it is a usually pain free process and you will remember almost nothing about the actual procedure.
Hope it all goes smoothly for you.