Questions about H1-B extension

Hi Brothers and Sisters,
The first term of my H1-B visa will get expired in June. I really appreciate your help if you can give me some info about the following questions:
1. When is the latest day to submit the application for the extension?
2. What documents should I and my employer supply? Where to submit the application?
3. Is there any form or document that I should sign?

Thanks a lot in advance.