If I join US ARMY,can I do 20 years and retire, snd also can I go to officr school?

If you join the Army you must display good conduct and be promoted to be able to continue your service to retirement. The lowest enlisted rank that may serve to 20 years is a SSG and the lowest officer rank is a MAJ. (these may have changed) So you can't just enlist and know for sure that you will be able to serve to retirement. Good luck!