Dear Gradschool, thanks for the kinds words. The truth is that I already finished college and grad school myself, so I went through the same things you are going through, now. There is really nothing that helps writing as much as actually doing it over and over again.
I had a good elementary and HS education which helps, and we spent much time going through all the rules of grammar and punctuation. However, over the years I forgot some of those rules, like how to make the correct plural and possessive forms of words which end in vowels, consonants or, most confusing, in the letter “s”. I sometimes go to online grammar sites to remember.
The best advice that I ever saw was a 2-page guide to writing which somebody put out in my college that reinterpreted the rules of punctuation. Instead of worrying about rules, it showed how each punctuation mark changes your tone of voice. Some whisper, some talk and some shout. You modulate your voice with the appropriate punctuation marks. That system opened my eyes. If I find that anybody has put it online, I will post it, Bli Neder.
But the most important thing is to proofread your work multiple times. I read that even pros do it 4-5 times. You will often catch run-on sentences, or missing punctuation marks that slur words together.
As far as vocabulary goes, the only real way to build it is to read and read. There are some good books I used for the SAT such as Norman Lewis’s Word Power for Better Meaning. These go into the Greek and Latin roots of many words, so you can figure out new words which you have not seen previously. He has good jokes and it is fun to read.
Hatzlacha Rabba with your career.