Technically to join the US Army all you need is:
- Be able to read, write, and speak sufficient English to understand the oath of enlistment and the pre-enlistment interview.
- Score high enough to qualify for the MOS (job) you are enlisting for.
BUT, to meet these requirements you will need a high school diploma or equivalent.
At this time, the US Army is usually only accepting recruits with a diploma or GED along with, at least, 15 college credits (in classes numbered 101 or higher). You aren't likely eligible to join the US Army without a GED.