I've never tried playing online league and was wondering how does it work ? Is there "manual" somewhere ? How often games take place ? Do I call plays or my coach does it ? Can I create and add my own plays ? What roster is being used in online league? Real or fictional?

It depends on who's created the online league. In online leagues the commissioner sims the weeks, but otherwise you run your team as normal... just with Export Team available so you can publish your team changes to the league. (The commish imports on a regular basis)