I have a better, slightly more difficult version, and an easy, not as good version.
Method 1: Download forge, from here, and then press the Windows key + R, and enter in
%appdata%. Press enter. Go into the .Minecraft folder. You should see a folder named "Mods." If you don't, right click inside the .Minecraft folder, and go to New > Folder, and name this folder "Mods." Now, you can put your mods in this folder. Mods come in the form of .jar files. To get started, try installing the Securitycraft Mod!
Method 2: Go to skydaz and download an installer for almost and mod you would like! :D