militaryrates.com lists the average yearly income for someone newly enlisted in the military at $19,000.
militaryrates.com lists the average yearly income for a new military officer at $36,000.
As for what branch of the military you might choose, there are many websites that give you information about the various service branches:
With regard to graduate training, you can decide to serve as a Military Psychologist after earning a Masters or Doctoral degree. A Military Psychologist focuses on military personnel and their families. Thus, a Military Psychologist might give psychological evaluations, treat mental and emotional disorders, and provide therapy. These services are provided in various locations, including the US, bases overseas, and aboard US Navy ships.