you are good, but your english probably isn`t perfect. you probably can`t understand english-language tv, read books in english, talk to native speakers easily, write letters without mistakes, etc.
you should never think your english is perfect. even if you are the best student in your class, always try to find your weak areas and work on them. when you`ve learned to speak english well, your problems will be quite small: punctuation, rarely used grammar structures, rare words, understanding "street language". right now, your problems are probably more basic: mistakes in pronunciation, small vocabulary, grammar problems with the present perfect tense and conditional structures.
use your english whenever you can
this is very, very important. the more you use english, the more you will want to learn it.
because english is so popular, you can use it everywhere. you can use yahoo to find english-language websites with interesting information, you can watch american cartoons, you can play adventure games on your computer, you can read interesting books in english, or you can do other things that we write about.
if you do these things, you will not only have fun and learn english. if you see that a new english word lets you understand your favorite tv show (or communicate with people, or beat a computer game), you will want to learn more words. so you will learn english more, use it more, learn it more, use it more... if you also use effective learning methods, your english will grow faster than you can imagine.