I joined the marines 22+ years ago--I was in MEPS 2 days after my 17th birthday. Back then you didn't need a highschool diploma.
Best thing is, you are so young that it doesn't really matter if you do or you don't. Seems like a big decision now, but in the scheme grande, who gives a shit? As long as you don't get shot, you'll soon forget about it. I did 10 years, been out for more than 12 years and its the furthest thing from my mind unless someone reminds me of it. I don't have anything from it I don't think. The main reason is that my dad, a Vietnam vet, was Director of Veterans Affairs for his county for a number of years and all these guys would come in wearing medals and limping and all claiming to be snipers and bayonet killers and he would look them up and they were truck mechanics or supply stockmen. So for me, it's more fitting to just let the past live in itself. I always thought I would have some stories to tell my kids, but I don't want to be lumped with the frauds.
But f you do want to join, start having some pride in yourself and puncuate and spell properly. You don't need to do pushups or run to get ready for bootcamp. It starts out pathetically easy. It's mental toughness that is required. Not a lot of it, but enough that it bothers some people. You just need two words running through your head to get through anything--Don't Quit. Your brain might start rationalizing reasons why it's not your fault and why you are getting it worse than anyone ever got it before you. And it will be complete crap. Don't Quit.
If you can take being treated like dirt, and you do that by understanding that YOU, formerly an intelligent and self reliant person that doesn't need anyone for anything, are now part of a machine that for the moment requires you NOT to think, you will be okay. After bootcamp you will suddenly be required to think again. During bootcamp you are just a piece of meat. You need to be a tough piece of meat, mentally. When you arrive, no matter how many people you've talked to, you don't know what is going on. You will be disoriented the entire time. There is no routine. All your personal belongings--including your hair--are gone. Your identity is gone. You are just a naked dude in a row of 60 other naked dudes and whatever you are is what you do. For 3 months you won't have a single thought but what you are going to do in the next 30 seconds. You'll feel like a tree that was just pulled up by the roots. You won't ever feel completely replanted again. After bootcamp you get 10 days and then you are off to your occupational school which is harder than bootcamp. And they treat your worse and much more is expected of you and everything moves at a faster pace--except you sometimes get weekends off. And you can smoke.
And then you get to your unit eventually--I spent a month on working parties at Pendleton waiting for a plane to Okinawa. And in your unit you are probably going to be shipped to Iraq or Afganistan. We went to Desert Shield with about 40% strength back in Aug '90. When the newly trained arrivals showed up in December I remember their eyes as big as dinner plates as we eagerly lumbered, disheveled, toward the trucks to divide them up. You'll feel like fresh meat again wherever you end up. Unless Obama fixes the world in the next year.
And then there is the issue of getting shot. A combat zone is a place you won't be able to avoid in the marines. The rules hover out of sight. They may pounce on you at any time, but they don't protect you like they do anywhere else. Anytime you engage the enemy you are at risk of being attacked by the rules afterwards. Also, you may find yourself with a moral dilema you may find hard to sort out. I personally have 3 or 4 events I wish had not have happened as they did. I guess killing people that you think don't really want to kill you and are going through the motions is different than killing people that are actively engaged in trying to kill you. Either way, you'll have some baggage somewhere.
I hope this lengthy discourse has been of some value. Like I said, you don't get shot, it doesn't really matter. Maybe join something where you get money for college. I joined the marines because of all the years I spent flipping through the Time Life World War II series. Those pictures in the Island Fighting book--nobody has bigger balls than someone that will jump out of a landing craft that has bullets drumming against the sides. I could not stand thinking someone was tougher than me on this planet. I did get to do landings, but not with people shooting at me. I didn't end up feeling disappointment.