This site tells you everything you need to know to get started and has what you need.
You will need WinRar or some program to open the files you download.
First you want to open your documents and find the electronics arts folder. Open the electronics art folder then find the Sims 3 folder and open it. Go to the link posted above and and download the Mods folder . You will find the Download under the setup folders and files in big letters you will see DOWNLOAD in blue. Click it and you will have your Mods folder ready.
Now you need to drag the Mods Folder into the Sims 3 Folder . You now have the mods folder ready for any Mods and CC .
To download you need to find what ever mod you want and download the file. When you got the file drag it into Mods/Packages and drop it in the packages folder. It should now show up in game when you play. :)
I hope this helps some i'm not the best at explaining things just thought I would give it a try.