It depends on which version of the program you're using.
And it depends on if you're trying to save as a DV-AVI or as one of the other dozens of AVI codecs.
But the basic move in version 11 is simply to go to Publish & Share/Computer/AVI.
If you're looking for more information on the basics of this program, be sure to check out my free 8 part Basic Training tutorials at Premiere Elements support site Muvipix.com.
Welcome to the forum.
In addition to Steve's directions, take a look at this FAQ Entry, for a bit of background on file formats and the CODEC's inside: http://forums.adobe.com/thread/440037?tstart=0
This is background reading, to learn what all can go into the AVI "wrapper," and perhaps help you choose the correct, or best options, when outputting your Timeline.
If you get stuck on which CODEC/Compression to use for your AVI, do not hesitate to ask here for specifics. When you do, let us know exactly what you want from that output file, i.e. Import into another Project for additional editing, e-mailing to friends and family, etc..
I also give a big +1 for Steve's Basic Training Series.