Can a person with the grammatical knowledge of your average 6th grader still be a good (not a great)

I could read your post just fine. Having a Facebook page with several hundred friends, it's not uncommon to try and read through someone's status 2 or 3 times before you actually figure out what it is they're trying to say. First, reading will certainly help you with your grammar and punctuation. Pay attention to how things are structured, where new paragraphs begin, comma placement, dialogue, etc. I know you don't want to hear about taking an English course but what if you were self taught at a very slow rate. An example would be for the next two to three weeks you're going to specifically just learn about the proper use of commas. Every day watch a new video on youtube about it, hit a few websites showing comma placement illustrations and perhaps find some free quizzes or exercises online that test your knowledge. At the end of those few weeks you will be all set on commas; no more issues with them! Then move on to another topic within the realm. Good luck!