Normally, my birthday is during Spring Break, so I usually relax and if my gift was a video game, I'd try playing it most of the day.
If it's not on Spring Break (and it hasn't been recently), I wake up, and go to school, and hear my birthday being said in the morning announcements.
"A very happy Nth (15th, 16th, 17th, 18th) birthday goes to Michael from all your friends!"
Then I go through my classes, hoping someone would remember it was my birthday. When I get home, I usually see my present somewhere, which I'd open and do stuff with.
Then, during the night (regardless of Spring Break or not), my parents bring out a cake (usually ice cream cake) and sing that song people sing on someone's birthday.
And, that's pretty much it, I usually go by my normal day.