Here is how you install mods, it's actually really simple.
1st. Make a
backup of all your saves/worlds, just in case.
2nd. Go into your Sims 3 folder, it will be under Electronic Arts , you can also just search it.
3rd. Inside the Sims 3 folder make another folder titled, "Mods"
4th.
Download this , it is a resource that allows you to run mods.
5th. Put that in the "Mods" folder.
6th. Inside the "Mods" folder, make another folder titled "Resources"
7th. After you do that, download the mod you want.
8th. The file will most likely download as .zip. If it does, unzip it. What I do on my Windows 8 is use 7-Zip. Right click the file, then 7-Zip, then extract files.
9th. After you extract it, put the extracted files in the "Resources" game.
10th. Load it up and have fun!