Please see the man page for sources.list(5) for the gory details on the correct format of your sources.list file.

In general, you will have one or more lines pointing to resources available. Each line will start wiTh either "deb" (binary packages) or "deb-src" (source code packages) followed by the base URL for the repository, then the release name and finally the various branches of the release that you want to use.

deb jessie main contrib non-free

The above line says "you can get binary packages from - and I want to get packages for Debian 8 (release name Jessie) and I want to use packages from the main, contrib, and non-free branches".

Linux Mint - base URL
Don't forget to add the appropriate Ubuntu sources as well!

Debian - base URL
Note that you can reference Debian releases either by release type (testing, stable, oldstable) or release name (stretch, jessie, wheezy). Ubuntu - base URL