For basic autocompletion, have a look at the files in %ProgramFiles%\Notepad++\plugins\APIs. It's basically just an XML file with keywords in. If you want calltips ("function parameters hint"), check out these instructions.
I've never found any more documentation, but cpp.xml has a calltip for fopen, while php.xml is quite complete.
The link provided by Mark no longer works, but you can go to:
Notpad++ 6.6.9
Settings -> Preferences -> Auto-Completion -> Enable auto-completion on each input.
I find it very annoying though, since a big autocomplete block is always coming up and I would just like to see autocomplete when I press tab or a key combination. I am fairly new to Notepad++ though. If you know of such a key combination, please feel free to reply. I found this question via Google, so we can always help others.
Check Enable auto-completion on each input. By default the radio button for Function completion gets checked, that will complete related function name as you type. But when you are editing something other than code, you can check for Word completion.
Check Function parameters hint on input, if you find it difficult to remember function parameters and their ordering.