Take a pre-ASVAB test, medical review, take the ASVAB, sent down with a counselor to choose a job type among many based on your ASVAB score. Verify you meet any initial requirements (clearance back ground check, color/vision requirements, any other medical requirements).

USMA, ROTC, OCS rigorous, get in shape, don't be fat. If you apply for any of these programs make sure you are complete on your applications. If you lie on anything they will find out and you will not like the consequences. Good luck.