First off, you must have a diploma or GED. They will also administer the ASVAB test that can determine what job you can get, it is just basic math, English, and other simple concepts. You must be able to pass a drug test and a medical exam, this just verifies there is nothing wrong with you medically. Don't lie or try to cheat. Next, you must be able to pass the minimum fitness standards for the Corps (They aren't hard, it's a 1.5 mile run, pullups, and situps). You will also get a background check to verify you don't have a criminal background. If you check all of these boxes, you're all good to go.
Also advise you speak with a recruiter in person to get a better understanding and all your concerns addressed