The Mint, Ubuntu, Medibuntu and Canonical repos are set up automatically in Linux Mint. If you insist on always having the newest software you can go to MintUpdate (the tool you should always use for updating) and check "safe" and "visible" next to level 4 and 5 updates BUT that's really not recommended - especially level 5 updates are outright dangerous to install.
In addition to that you can uncomment some repos in sources.lst. Type sudo gedit /etc/apt/sources.lst in the terminal and uncomment anything you like (you will probably not need the source repos).
War does not determine who is right - only who is left. - Bertrand Russell