How do I donwload mods

First, you need to install Forge. It's pretty easy - There are installers available, and they do the job for you.
After that, you need to install the mods. Some of the mods provide instructions of how to install the mods, and that is very important. Some mods may require others to function properly.
(In general, most of the mods are installed by putting them in a folder with the name "mods", inside of your ".minecraft" folder. To access it, type "%appdata%" on your computer search bar and open the ".minecraft" folder. If there is no "mods" folder, you can create one.)
After installing the mods, launch minecraft and you're good to go.