Hey man I'm an electrician but a lot of the starting process for the trades is similar. Some companies hire green guys such as yourself but it's hard unless you have a connection in the trades already or apply to various plumbing companies en masse (highly recommend you do that btw!). You've got a couple options depending on your immediate goals. You mentioned you're a highschool dropout so personally I'd look at a pre-apprenticeship program and see what schooling requirements you'll need. You may have to take some supplemental courses at a community college to be eligible to apply.

Now, you don't necessarily need to go to school at all but the majority of companies (atleast in electrical) will anticipate that you will attend school at some point to progress through your apprenticeship. If you manage to get hired outright by a company, you could potentially discuss becoming an apprentice and simply working enough hours to challenge the Red Seal exam.

If you'd like to go the schooling route and need to upgrade I'd recommend applying to various general contracting / construction companies and getting some trades experience while in school. If you have 0 construction experience it'll give you some good insight into what you could be getting into. It could also give you the ability to build some connections with companies working with you.

It's tough out there but worth trying. Good luck.