Go to Publish & Share, select Computer, then select AVI as the output option.
I assume you're looking for a DV-AVI file. (There are literally thousands of possible AVI codec options.) If so, make sure you pick the widescreen preset if you're working widescreen.
Some other reading
Online User Guide http://help.adobe.com/en_US/premiereelements/using/index.html
-Page to download current PDF http://helpx.adobe.com/premiere-elements.html
Importing Video http://forums.adobe.com/thread/1065281
-and project settings http://forums.adobe.com/thread/1112086
Saving & Sharing http://forums.adobe.com/thread/1137128
-Sharing to DVD or BluRay http://forums.adobe.com/thread/1137645
-Sharing for Movies http://forums.adobe.com/thread/1051093
-Sharing for Computer http://forums.adobe.com/thread/1058237
Steve's Basic Training Tutorials... steps are the same for several versions