If you installed i3 using your distribution’s tools, you should have a man page available. Please read it completely before asking any questions. You can view the man page using the following command:
$ man i3
If there is no manpage, please kick your distribution’s package maintainer. You can view an HTML version of the manpage online.
If you are a developer and would like to get into i3’s source code, please read "How to hack i3" first.