Reddit reviews Grammar as Science (The MIT Press)

Mostly Grammar as Science, which seemed to put things a little simply. Plus we didn't actually have to buy the book, she just put scanned pdfs up on the course website for the chapters we used.

My school doesn't really have a very strong undergrad program, which is part of why I'm minoring instead of my original plan to major. Our Graduate program is apparently pretty good though, apparently enough so that they rarely accept any of our undergrads.

Regardless of what your program's focus is, my best advice is to bone up on syntax and morphology. Of course, if you're studying more practical/clinical stuff like language acquisition, phonology will also be indispensable.

Either way, you're definitely going to need an understanding of the basic principles of syntax. My 400-level class used Grammar as Science by Richard Larson. I liked the diagrams. Another class I TA'd used the O'Grady textbook, which someone else mentioned here, and I'd definitely recommend that, with the caveat that it's designed for 100-level stuff. So start with O'Grady, then move on to other things.
A suggestion: are you into conlanging at all? Because that's a great way to get your feet wet. It's also super fun.