To start inserting, you can press "i"... when you're done typing, you can press ESC to go back to command mode, where you can type commands starting with a colon (":")... i.e. to save and quit...
:wq

(the colon means you're giving a command, w means "write the file" and "q" means quit)

This assumes you gave the filename at the command line, if you only launched vi without a filename (i.e.: only just typed "vi" ) you can insert text and finally write out the file with a filename like this:

:w /etc/apt/sources.list.d/mymoves.list

and then later quit with:

:q

If you've made a horrible, horrible mistake and you want to quit without writing the file without the changes you've made since loading, you can quit with a panic, like so:

:q!

This will tell vi to just quit without writing anything.

I hope this helped.