How should I decide on what style of site design I should use for a project?

If you're trying to hack something together for yourself, perhaps to make money or learn something, the layout will likely come from something you found on the net, which you'll customize. (eg, bootstrap layouts, etc...). It is unlikely you'll do any wire framing or mood boards.

If you're working with a team in a more corporate environment, typically you'll get the design from a graphics person and they'll have you implement it. Chances are you'll be implementing it not from scratch, but from a similar layout someone else coded a while back.

So, I would recco finding existing templates that get you kinda of where you want to go, but you make major modifications to arrive at what you conceptually like. For instance, maybe https://startbootstrap.com/template-overviews/sb-admin/ you want to make a dashboard, but you need a grid of 4 charts and don't want the alerts on top. You also need to reskin the whole thing to make sure the colors match your company's colors.

My 2 cents.

Here's a website where people post their Sketch files: https://www.sketchappsources.com/, you could take some of those mockups for inspiration, modify them, code them out.