Whoa... slow down on the BIG dreams kid. Not trying to kill your dreams, but lets tackle one thing at a time. How old are you?

First, set yourself some physical fitness goals. Lets start with the run time. You will absolutely want to get that 3 mile run time down to under 20 minutes. As for the PFT, you do those twice a year. In boot camp you'll do an initial strength test to see if you can actually hack it, then you do a PFT sometime before you graduate. You do a PFT twice a year regardless of your MOS. I would skip the treadmill and run out in the open or find yourself a track. Your ultimate goal with the run is to get that under 18:00. I know it sounds hard, but it's not. You just gotta keep pushing yourself and never quit. It takes time, so don't be expecting any results over night.

If your gym has a pull-up bar, see how many you can do. You can do overhand or underhand. You just gotta start from a dead hang and go all the way up, bringing your chin above the bar and then going right back down to a dead hang. You'll wanna end up doing 20. Don't fret if you can't at first. You'll just have to work on it. Start with doing lat pulldowns if you can't do 20.

Can you do crunches? Can you do 100 proper crunches in less than 2 minutes? If not, get cracking on them.

Seriously, just start with all the above in trying to get a perfect score for each one: http://www.military.com/military-fitness/marine-corps-fitness-requirements/usmc-pft-charts

Second, I'd consider talking to a recruiter about all this. He or she will be your best bet in answering all of your questions. Also, it's not all shoot'em up GI Joe high speed shit you see in the movies. There is a lot of bullshit they don't show you or tell you about. Such as cleaning your rifle, over and over and over. Lots of room, uniform and equipment inspections. Lots of doing shit you will hate doing. Lots of doing shit you hate doing now.