El W; I'll try to keep this as basic as I can for now, if you want to talk submarines- we can do that. But not now.
Figure out what you want to do and what you want to be. Keep your mind open, you don't even have to join the military at this point.
Next, figure out your goals. Where do you want to wind up when it gets close to your retirement? Keep a little wiggle room in your plans but definitely start to think about long-term goals. You want to be a what? Why do you want that? What other options are out there?
If you are sold on that military career, do you want to be an officer or enlisted? Both have their plusses and minuses. Both are separated by a "glass wall", you can't really be both (there's the concept of a Warrant Officer, but this career path tends to be very tricky to navigate) but both can be rewarding in their own way.
Also, keep in mind that at its core, a job in the military is a Federal job. Some of this can be a headache, they tell you exactly what is expected and what isn't tolerated and there's very little room to innovate or experiment.