I'd add, you can improve your language through reading. This means reading books, newspapers and magazines that are more difficult than average. That is:
* The New York Times or Washington Post rather than The New York Post, Washington Times or Boston Herald.
* Smithsonian, National Geographic or even Sports Illustrated rather than People. The examples I've given are well-written and SI in particular has excellent symbolism. There are other well-written magazines out there, of course; choose whatever appeals to you.
* Classics of literature or at least well-written works (think Jane Austen, Thomas Hardy or even newer books such as Harry Potter, believe it or not, rather than a sports biography as told to some hack Hollywood writer). These don't have to be boring. Pick something that you like and read about it - science fiction, history, sports, mysteries, etc.
Read, read, read. And remember what you read, so select something that interests you and something to which you'll pay attention. Just skimming isn't going to cut it; you're going to have to spend some time on details. I'm not suggesting doing book reports on any of these things, but I am suggesting sitting down and really focusing (no radio playing in the background). The summer is a great time for this. Take a chair to your back yard, slather on some sunscreen and spend some time reading.
Some basic stuff from your post:
* punctuation - love it or hate it, it gets across mood, emphasis and meaning (we're versus were). You need to use it.
* capitalization - you're typing as if you're chatting, and school isn't like that. You need to capitalize sentences, names and titles.
* spelling - this goes along with paying attention to what you're reading. Everyone makes typos and that's understandable. Don't beat yourself up over those. However, you should spend a little time proofreading what you write. This doesn't just mean using a spell-checker; it also means rereading what you write in order to be sure that it's correct. Have a friend help you. Even a friend who isn't so good in English may be able to point out basic instances wherein what is written doesn't make perfect sense or look like a word. Also check dictionaries. If you don't have one at home, use
www.dictionary.com.
Two other things
* meaning - this goes along with rereading what you write before submitting it, whether online or in a school essay, etc. Make sure you're getting across what you want to get across. A trick I use is to read what I've written out loud. If what I've written is something I stumble on when I read it aloud, that tells me that the structure is awkward.
* symbolism - you mentioned this, so I take it you are learning something about metaphors and the like. This will partly come from reading, but it will also come from usage, as Setanta says. If you find yourself saying things like
CDs are like boomerangs - they always go back to their starting points, you're speaking metaphorically. You may find it lame to say that (hey, cut me some slack, I just made it up) but if you begin to think of the similarities between things, people, expressions, etc., that's half the battle.
Best of luck to you - good English writing is a skill that will stand you in good stead in nearly every white-collar profession.