Lisa's answer is an excellent response to your question. The only thing I would add is that you should first decide whether you are going to college or advanced education beyond high school. If you are not going to college, I would recommend enlisting in the services and getting your professional training for your assigned career field. Depending on your grades and ASVAB scores, you may be able to choose the career field in which you will serve (in the military it's called a MOS -- military occupational specialty), such as aviation mechanic, communications, infantry, artillery, hospital tech, etc). The military will provide extensive training in your assigned career field. Enlisting in the military is a great first step out of high school because you learn discipline, job skills, get great benefits, and see different places in the USA and world. Enlisted members start as an E-1 and rise up to the highest level as an E-9 (in the Marine Corps, an E-1 is a Private, and an E-9 is a Sergeant Major or Master Gunnery Sergeant). Enlisted members comprise a little less than 80% of the active duty military.
If you want to go to college you should consider the various opportunities that the military offers, such as ROTC programs, academies, and military related scholarships. Many scholarship are offered by military related groups. If you enter the military with a college degree you may qualify to be commissioned as an officer, which starts as an O-1 and runs through O-10 (Second Lieutenant or Ensign through General). Some fields, such as lawyers, doctors/nurses, and others are only open to officers. Also, you should be aware that the various services offer opportunities for enlisted members to become "warrant officers," but usually only after years of top-notch service and schooling.
Whichever you choose, starting your life after school is a great choice and will serve life-long lessons. Less than 1/2 percent of Americans currently serve on active duty military. About 7% of Americans have served in the military at any one time in their lives. I am proud that I chose to serve as a US Marine, and am just as proud of my brothers and sisters in arms who have served in the Army, Air Force, Navy and Coast Guard. Best wishes