It depends how old you are, and what you want to do.

If you already have most of high school credits, you best option is probably to just complete those courses and get your diploma. Get your transcripts and sign up for some online classes. They're free.

If you intend to go to college, you don't need a diploma. You can apply as a mature student after 19 by writing an English and mathematics exam. If you pass, you are admitted. Of course, you do need to be able to pass them. So maybe you want to take that grade 12 math course like I mentioned above.

Universities also have programs to admit those, usually over 21, without high school diplomas. It usually consists of taking a standard undergraduate course over the summer. if you do well, you're admitted for undergraduate studies.

I'll probably get flamed for this last one. But if you're well into your 20s or older, the honest answer is that you can likely get away with being dishonest. While some employers may want proof with a transcript for an 18 year old with no work history who claims to have finished high school, almost none check if some 27 year old has their OSSD when they apply to a warehouse job or whatever. You either pass muster or you don't by your performance, presentation and employment history.