There are also other free sites with exercises and tests if you look for them. At the end of the day, it is your willingness to read, write, speak and listen - and to practice all of these - that matters.
If you want to be an English teacher and wants it free, it would really require greater effort on your part: Improving your own English and imparting them to others. We cannot give what we do not have, so try to check every English site you can access and study every book you can get your hands on. They give you a lot of useful insights, not to mention help.