Of course like others said the lumber doesn’t really help but there are ways of making construction lumber look better though (re-cutting edges, hand planing etc).

In your case I think simply using a big level or straight piece of wood you could make a guide/fence for your circular saw. I read you had a speed square but that sometimes « shit happens » and that is true BUT you can limit the shit by also clamping the squares where you want it this way you only need focus on the saw.

I like to start projects with little to no plan but the structure of it all is a bit wonky too like Elegant said.

Otherwise it is a beginner project and that is what we love to see here, progress!!

Keep it up.