If your track(s) have added effects, or trails that last longer than the duration of the last clip, use the mouse to click and drag across the time ruler within the duration of time you want to export. For effects like delay and reverb, adding an extra couple seconds after the last clip will usually help!
Go to File > Export > Audio.
Select the location where you would like this file to save.
Add a file name, and choose a file type from the menu below.
Make sure the source category is set to Entire Mix, or you can choose any of a host of parameters to reflect your needs.
Click Export, and check the selected location for your final product when exporting is complete.