Hi Daniele, welcome to the forums.
Notepad++ comes with an autocomplete option in the program settings. You have to go to Settings -> Preferences -> Backup/Auto-Completion, and enable the autocompletion options. This system is not as accurate as auto-complete in other development environments, because the suggestions don’t really differentiate between functions and variables. It’s still good to avoid typos, though.
I know there used to be a Corona-specific plugin for autocompletion in Notepad++, but I was never able to find it. There is a Lua-friendly xml file to improve its autocompletion, which is here: http://www.retroaffect.com/blog/190/