If you're just looking for clothes, hair, and houses, I think http://thesims2.ea.com/ is the best, and probably the easiest. The Exchange has a ton of downloads made by users.
When downloading off of http://thesims2.ea.com/ (example through the Exchange or the Store) you will automatically receive a download message or a blue button that pops up asking you to *install*. Click install and it will install directly into your game. Thats it! :)
To actually access the content you download you have to make sure you have enabled *custom content* in your game.
Click "Options" and then click "Game Options- the picture of gears" and in the upper left corner click to enable custom content at start up or whatever it says..
You may have to exit and reload your game, but from now on every time you start your game it will ask you if you want to enable your custom content..ie: your downloads. Say yes.
***NOTE: EA warns that custom content from fan sites can mess up your game, but personally Ive never had any trouble using content from http://thesims2.ea.com/ ***
If you don't automatically receive the button to install the exchange items, you may have to download the EA Download Manager or (EADM). This is cool anyways b/c you receive the Sims 2 Store Edition which has random extras.
Anyway.. the EADM is an application built to manage your downloaded content from EA.com. (Primarily the The Sims 2 Store items)
Go here: http://thesims2store.ea.com/help.html and click the link: *download the EADM*
Hopefully that helps! :)