James provided a lot of good information on the pay for military service. I would like to add to his answer.
If you joined right out of high school or without a college degree, then you will come in as an E-1 pay grade. Depending on the service you join (Air Force, Army, Navy, Marines) and the AFSC/MOS (infantry, intelligence, military police, etc) you can also qualify for an enlistment bonus. The higher-demand jobs, can usually come with a bonus. A recruiter would have the latest information on bonuses. Also, the length of your enlistment contract can also change your pay grade. For example, the Air Force would bring you in at the pay grade of E-1 (the rank of Airman Basic) if you signed a four-year contract. If you signed a six-year contract, they would bring you in as an E-2 (the rank of Airman).
Another thing to keep in mind is your living expenses while in the military. As a junior enlisted troop, you will be living in the dorms and on a meal plan. This means you won't be paying rent or for food, which are normally a person's two highest expenses at this point in their career. If you are (or get) married, you won't live in the dorms anymore. You will move out to either base housing or can live off-base. The military will give you a housing allowance to help cover the cost of rent, food, utilities, etc.
Lastly, you can come into the military as an officer (Second Lieutenant or Ensign). To do this, you either need to go through 1) a service academy like West Point, Naval Academy, or the Air Force Academy, 2) a regular college that offers an ROTC program (Army ROTC, Navy ROTC, Air Force ROTC), or 3) go to college and get a degree and then apply for Officer Candidate School (Officer Training School). Joining the military as an officer has its own ups and downs, but always comes with better pay.
As James mentioned, there are also lots of other benefits besides the pay that you should take into account like the GI Bill to help pay for college or a VA-backed loan to help you buy a house. You will also have health insurance for you and your family while in the military. Most bases also have a Base Exchange (kind of like a base Walmart) where you can buy groceries and other things at a cheaper rate than what you can off base. All of these things can reduce your costs, which have the effect of making your money go further.
Daniel recommends the following next steps:
Assuming you'll start as an E-1 (the lowest enlisted rank), you'll make just under $1,700. You can also get what is called COLA (Cost of Living Allowance), which would increase your pay, but it depends on where you are stationed.
You will get paid more as you get promoted and depending on your time in service. Assuming you make it to E-4 in 2 years you'll be making ~$2,300.
Keep in mind you'll also get benefits when you get out like the GI Bill and VA Loans if you buy a house.
James recommends the following next steps: