How to install mods? Same as you always do: place them in your mods folder, which you can find in your .minecraft folder (%appdata%\.minecraft on windows).

Unless you mean how to install a mod loader. That's also pretty easy: go to the Forge website, download either the latest or recommended version and then doubleclick on the downloaded jarfile. If your Java installation is correct then it will start the installer. Just follow the instructions and you should be fine.