Study or travel to an English-speaking country. Of course the best way to immerse yourself in the English language is to live in an all English environment. You'll find yourself surrounded with the language everywhere you turn. It can be a more difficult and expensive choice, but you may find it to be a very valuable and helpful experience.
Other ideas. Take an English class, join an English group, find a tutor, read English magazines. Do whatever you can to immerse yourself in the language.
Monitor yourself
This means to pay attention to how you use English. Think (a little) about how you are going to say something before it comes out of your mouth. But don't think about all or the problems you have in English at the same time. Just choose something your know you are having difficulty with and work on using it correctly. For example, if you know that you have problems remembering to use a, an, and the, spend a few days trying to put them into sentences as you speak. Don't worry about anything else. Concentrate on that. When you feel that you are improving, choose something else to work on. Just keep trying and little by little, your English will get better everyday.