Welcome to the forums.
The basic thing is to practice as much as you can, and to read as much as you can.
However, there are tests on this site that you can use to check your English, a good Grammar Glossary to tell you about grammar, and in Resources you will find links to other sites.
You are very welcome to post any problems for someone to answer, or to post your writing in Editing and Writing for comments.
- For Teachers