The only sure way is simply to practice. Language does not come easy and as long as you put in the effort, you will improve. How fast you improve depends on how much you practice. The more practice you get, the faster you should see yourself improving. Also, when you are practicing, try to evaluate your own skills. For example, when writing, try to find the errors in your own writing before offering it to a native speaker to correct it for you. If you get in this habit, you'll become better at spotting grammatical or spelling errors in yours or others writings, which will make you a better student as well. :)

You can improve your English by reading,speaking,and listening.