How to get an equivalent of a high school diploma in order to join U.S. army / the military?

Get a G.E.D. and go to community college and get 15 college credits that CAN BE TRANSFERABLE TO A UNIVERSITY. This is the only way you can be considered to be equal to someone with a High School Diploma short of getting one yourself. You can't just have the G.E.D. it's not impossible to get in with just a G.E.D. but you'll be put way at the bottom of the priority list. So if you can get into a community college and get those 15 credits you'll be fine