I didn’t really learn how to write well until I went to law school and a very patient professor worked with me to write better. Oddly enough, I got a 5/5 on my AP English test in high school, so if that’s your goal, you don’t have to write too well. With both AP English and law school, the secret was establishing what I wanted to say clearly and quickly before moving on to the next sentence. Natamora is giving you some really good advice about that – clarity is better than worrying too much about rules. Your goal should always be making it easy as possible for the reader to understand what you’re trying to say.
vikaTae is also correct by saying different works require different styles. A message board post is much for informal than, say, a patent application. Even if you end up with top marks in AP English, you can still be a bad writer.