I know that there are fundamental rules to layouts just like photography, but I'm mostly clueless. So I think that's the area I really want to focus. I'm mostly interested in design for print media (posters, brochures, etc).
You should learn Quark, too. I would imagine Indesign is similar, but I've never actually used it. It does depend if you want to work for yourself (then you generally use whichever you like), since if you work for someone else you have to use what they want.
Most of a good layout is from experience than anything else. When I started writing for the university magazine, they had a very by the book kind of layout, which really is surprisingly boring to look at when you just go by the general rules of thumb. When the layout was mostly being done by more art focused people who didn't know the layout rules, it actually ended up looking better because they were more focused on whether or not it looked good then following the rules of thumb the communications people learn in layout courses.
Knowing the rules and conventions is quite helpful, but being able to look at a layout and decide if it looks good or not is also quite important. Sometime violating a couple conventions does look much better, while meticulously following them comes off as rather awful.
It also depends, again, if you work for someone else, they might have certain conventions or style sheets you need to follow. If you work freelance, you need to give the customer what they want. So it does depend what you want to with it.