Most of what I just said about spelling applies to grammar as well, although there is one slight difference: most people, including your potential employers, are not masters of grammar. You can screw up a little here and there, and it’ll go unnoticed. For instance, should you say “if I were a rich man” or “if I was a rich man”? Unless you’re applying for a copy editor job at a newspaper, it’s not a big deal, because copy editors are just about the only people in the world who know every grammatical nuance of the English language (by the way, the answer is “were” – I used to be a copy editor). On the other hand, if you misspell “calendar,” that’ll jump out at everyone who sees your resume.
If you don’t know the difference between “their” and “there,” “its” and “it’s” and “your” and “you’re,” then don’t leave college yet.