Make sure to check off 'Enable Custom Content and Mods' and 'Script Mods Allowed' depending on what you've chosen to install. Once you're in game, you'll also need to enable mods in your game settings by going to your Options menu under the Other tab.
Your mods folder is located in: Documents/Electronic Arts/The Sims 4/Mods. The game should automatically have created one for you after you start up the game for the first time. To install Sims 4 mods, you can drag and drop the mod folders you've downloaded into your Sims 4 'Mods' folder. How do you install mods for The Sims 4? Luckily it's pretty simple, especially if you only want to make a couple tweaks.
