If you need extra help (or are just a very motivated learner!), here are some excellent resources:

I. General English practice (grammar, vocabulary, pronunciation, reading exercises, etc.)

An OUTSTANDING website. You can listen to interesting news stories on a variety of topics, learn new vocabulary, practice grammar, and find out what's going on in the world.
CHECK IT OUT!
has all kinds of games, puzzles, exercises and quizzes to help you learn grammar, vocabulary and pronunciation. Some of the games are fun, and a lot better use of your time than YouTube :-)

II. Vocabulary

This site has links to vocabulary-learning services. You can subscribe to "word a day" or "idiom of the day" which can be sent to your e-mail account. Just think, by learning only one word a day during the school year, you'll know 180 new words by the end of the year!

GREAT resource for helping you learn the Academic Word List.

(There are additional resources on the "SAT & TOEFL Resources" page....)

III. Listening Skills

There are "easy', "medium" and "difficult" listening exercises, with both comprehension quizzes and vocabulary quizzes included. Excellent practice.