I got a selective service thing in the mail and I have no idea what to do?

It sounds like what you got is just a reminder with a little advertising for the military recruiters on the side. Nothing has "changed" and you haven't even submitted your information yet, so there is not much you can do about the letter. Just go ahead and register on your eighteenth birthday (or within the time-frame allowed) including the information on your diabetic. Back in my day we got (the infamous) draft card to carry in our wallet. I don't know if they are even issued anymore. But once you are registered you should be okay.