You have your life more together than a lot of people and seem to know what options are available. If you go to school, what would you study? Your first step would be getting an associates degree from a local community college. Then you could transfer to a 4 year university to receive a bachelors with 2 more years of study. This is the best option if you know what you want to study and how to get a job in that field after graduating.
Military is an option, its a steady job and veterans get some benefits, like free tuition to public universities and a preference in hiring for federal and some state jobs. This isn't a bad option, there are a lot of people who join the military when they are in a financial pickle, but they have a lot of rules to follow and require incredibly long hours, maybe living on a base or going overseas.
As for a union, thats controversial. It depends on the employer you work for. In general, a union can step in and prevent you from being fired unjustly, but they do take some money out of your pay checks.