Q: how do I convert an .aiff file to .mp3?

iTunes: How to convert a song to a different file format - http://support.apple.com/kb/HT1550 - including information about different formats and discussion about compression.

iTunes 12 for Mac: Save a copy of a song in a new file format - http://support.apple.com/kb/PH19509

I use an older computer.  Isn't there a "File" item in the menu bar at the top of your computer screen?  Not in the iTunes window but at the top of the screen itself.

It sounds like you have iTunes in Full Screen Mode.  To access the File menu from there, run your cursor to the top of the screen and wait about a second until the menubar drops down.  You can also CTRL-Click the title of the song you want to convert.  This will require that the iTunes Import settings are set to mp3.

Good luck

srb

Boboser wrote:

In the latest iTunes there is no menu bar.

Command Control F

I'm glad it helped.

srb

Many thanks.