The command shift-j or shift-k will select to the left or right of your curser, and then you can use the Delete key to delete the selection, or cmd-t to trim all but the selected region.
Once you have made your trims, make sure the "Project Rate" field in the lower left hand side of the page is set to 22050 Hz, and go to (File > Export). From the page that pops up, select MP3 files for the format, and then click the options button. Set the Bit rate mode to Constant, and the Quality to 24kbps.
Click OK and then select the location to save your mp3 file.