Reddit mentions of Longman Student Grammar of Spoken and Written English

I've heard this before that there are no accrediting bodies for online TEFL certificates. I began one through ITTT and was disappointed by my experience, but I had a lot more knowledge going into it and ultimately went for a master's degree, so I wouldn't count my experience as typical. However, your abilities aren't going to improve from just doing a TEFL certificate. You gotta get into a classroom and pay your dues like all teachers. The things that become instinctual do so over time. There's no shortcut for it, but you can supplement your training with the wealth of materials out there for teachers. Two pretty decent texts on how people learn language (theory) and how you can apply that theory (instruction) are Theories on Second Language Acquisition and Communicative Language Teaching in Action. VanPatten also came out with an updated book called While We're on the Topic. I would also suggest the Longman Student Grammar of Spoken and Written English. If you read through all these, you'll be more prepared for instructing in a classroom than any TEFL certificate will prepare you for. Indeed, you'll also figure out that a lot of certification content excludes relevant research from the field.

As someone who also recently got into Linguistics at the university level, I can sympathize. Since my degree isn't Linguistics I didn't need to take a language for two years, but I would (personally) go for either Sanskrit or Greek.

General Advice: As someone else said, definitely work on IPA and sentence diagraming/sentence trees. Both are (in my opinion) extremely fun and interesting. I give this link to anyone asking for help with IPA. A TA of mine sent it to me, and it has been invaluable. Sentence diagramming can be tricky to pick up. One of the textbooks I used was the Longman Student Grammar and it is packed with information on various parts of English syntax. For 30 bucks on Amazon, it's definitely worth it (unless one of your classes uses a similar text, in which case, I would suggest waiting because textbooks are way too expensive).

Other than that, my only other suggestion is to get to know your Linguistics professors. Applied Linguistics is such a huge field (I assume most of your professors will belong to it), and each one of them I guarantee has a different specialty. Two of my professors specialize in speech pathology, one specializes in corpus research, another in syntax, and there are more that I won't list. Figure out what you really enjoy doing since it's so broad, and try to focus in on that (while obviously getting a broad knowledge of the other fields). Best of luck!

edit: I also meant to say, try to get involved in an internship with the department at some point in your college career. Much of linguistics is research, and having a research assistant internship on a future resume would stand out.