Maybe you should have paid attention, after all. Sorry, there's no over-night fix for this problem. You'll just have to go through the grammar tutorials, do the exercises, correct your errors and practice what you learn. There's another thing that your English teacher probably told you about writing. You can help yourself learn to write better by reading.
Good writers are almost always avid readers.
Read everything you can get your hands on. Read books, magazines, novels, short stories, poems, newspapers, matchbook covers. The more you read, the better. The more you read, the better you will understand what you are going to learn in the tutorials. Set aside some time every day and read for the pleasure of reading.