If you're in the U.S., you need to get your GED (or whatever high school equivalency is offered in your state) to have a decent shot at any career (at 18, it's probably not too late to "drop back in" to high school, either, if you can stomach it). Ask about it in your local library or community college.
Once you get your GED, you can apply for any job that requires a high school diploma. The armed forces accepts a limited amount of applicants with GEDs. If the armed forces or university do not appeal to you, you can go to your local union hall and inquire about apprenticeships, which pay you while you learn a skilled trade. Community colleges have vocational degrees (CNA, culinary arts, dental hygiene, veterinary technician, EMT, paralegal) that might interest you.
Otherwise, you're pretty much limited to warehouse, grocery checkout, custodial and fast food jobs. There is not much need for unskilled labor in the U.S.