Is there someone who help me to understand how this site works?
If you use the buttons at the top, you can go to the various sections:
Testing: you can test your knowledge with various types of language quizzes and exercises.
Reference: This area contains a grammar glossary, dictionaries of phrasal verbs and idioms, etc.
The forum is a place where you can ask teachers, people keen on language and other learners questions or discuss language-ralted topics. (best to register)
Etc- Just explore a bit.