A couple of the hoops you will have to jump through are the ASVAB and MEPS.
The ASVAB is a computer based test to see what kind of job you would be good at. Some jobs require a higher score than others. There are study guides and practice tests all over the internet, or your recruiter can get you started.
MEPS is a medical screening. They'll make sure you're healthy enough for the military, you meet the height and weight standard, and you're not on drugs.
You're going to have to meet a minimum standard for run time, pull-ups, ammo can lifts, and crunches. It's called the IST or initial strength test for the Marines. Military.com has a good breakdown of the current standards.
Rally point is an online forum for military related issues. You might want to check it out if you have more specific questions about the life, or a particular job.
Lisa recommends the following next steps:
- Talk to a recruiter
- Make sure you can pass the IST
- Study for and take the ASVAB
- Research a job or field