How do you dowload cc (hair,clothing, ect) in The Sims 3?

CC in TS3 comes in two different formats. Which one provided is up to the creator, some provide both for each item and let us decide which one we want (in this case we choose one or the other, not both).

Sims3packs are to be downloaded, decompressed if they arrive in a zip, rar, or 7z format, and the file(s) placed into the Downloads folder in the TS3 user game folder in Documents. The Launcher reads them on startup and from there they can be selected to Install. It is not necessary to keep the sims3pack files in Downloads after they have been installed, most of us prefer to store them elsewhere to give the Launcher less to worry about going forward.

Package files are to be downloaded, decompressed if they arrive in a zip, rar, or 7z format, and the file(s) placed into Mods\Packages. The Framework must in place first to work with these kinds of files, but that's all there is to installing them once the setup is in place. The Launcher is not involved at all.
http://modthesims.info/wiki.php?title=Game_Help%3AInstalling_Sims_3_Package_Files%2FSetup_and_Files

Crinrict has written a helpful guide (yet another one) to installing custom content here:

https://sims3.crinrict.com/en/2011/01/faq-how-do-i-install-custom-content-mods.html

One thing to note is that many sims3packs come with hidden cc items, some of which can be harmful to your game. Players can examine packs with Custard to identify extra files attached to sims, houses, or worlds. Custard won't find everything, though, and you may not want to learn how to use it right now, so the easiest way to protect your game is to install any new pack into a clean folder first to check it out.

Pull your existing TS3 folder out of Documents and onto your desktop and launch the game again, and a new folder will spawn. You can follow crinrict's guide to installing the pack into your clean folder and then scroll through the "installed content" tab in the launcher; look for any extra files besides the one(s) you intended to install. If you see any extra files, there are ways to clean the pack, but you might just want to skip that step for now. It's better to find out about bad cc before it infects your game.

Edit: Or you can do what @igazor says. But still, it's best to be careful when dealing with sims3packs from creators whose work you're not familiar with to trust.