Become a developer
Join community
You can join fcitx-dev on googlegroups first. And if you have some complex question want to get support directly, feel free to contact with User:Weng Xuetian, you can find his email and gtalk there.
Where to start
Fcitx have doxygen, you can compile it and run
make doc
For input method, read QuWei is good start, since it's very simple.