How can I fix the margin of my page to sit correctly on the screen instead of snapping too low?

Question

A little embarrassing, because I'm a newbie in the world of HTML and CSS specifically, but I'm practicing applying what I do know.

Basically, I've added all sorts of fun stuff to the original base code, and I guess somewhere along the way, something got messed up and now, whenever I load the page, it snaps down a little too low.

I've come to realize that this can be temporarily fixed when I make my browser smaller, and then maximize it to fit the whole screen of my desktop again, or when I zoom out and zoom back in to 100% within my browser. SOMETIMES (rarely) when I reload the page, it'll finally fit and you can see the entire page the way you're supposed to. Otherwise, just loading it regularly has it snapping down an inch or so below my toolbar, which is an issue especially since I can't add stuff to the bottom of the page without it getting eaten up by my toolbar.

I've tried messing within the code (and when it didn't have any positive result, I reverted back to the original code it's working on) but again, I'm a newbie and the code likely looks like a huge mess to anyone who actually knows what they're doing, unfortunately.