First, you need to install RAR file to unzip the the files. You also need to create a download folder in my documents, ea game, sims 2 if it's not there already.
Here is the link to install RAR, it's completely free.
To install WinRAR, download the file to your computer by clicking on the link on the WinRAR site's download page. Once it has finished, double-click on the file to run the installer. You should just be able to click next, next, next, etc. to install WinRAR. At the end, click Finish to complete the installation. You will most likely want the top link to the Graphic and Command Line, Trial version.
http://www.rarlabs.com/download.htm
Once you have this, then you will need to make sure you download the mesh of the item first. The creator will let you know if the mesh is needed (they will most likely include it in the download or attach a link to it)
Click the icon to download the object, it will come up with a box asking to save or open. Click "Open" then it RAR will appear, click "extract item" at top of screen, on the right side of the box will appear files, click "my documents" then click "EA Games" then click "sims 2" then click "download" then OK.
In your game you may have to enable custom content. (not sure what sims 2 game you have) if that's not clicked, once you do enable custom content, back out of your game and re-start it.
Most creators will let you know above the content if you need to have CEP installed. It's always a good idea anyway, and it's free. CEP is not needed for hair or clothes.
http://freya.modthesims2.com/showthread.php?t=92541
Let me know if you need more help. My IM and Email are open in my profile.