Hi Joey
First off, make sure that all your instrument/vocal busses (and any tracks that aren't being sent to a bus) are sent to a single 'master' bus; this bus should be set as the
only bus (or track) outputting to your soundcard.
Next, click on an empty spot in the
Track View and hit
Ctrl+A to select everything in your project.
In the
File > Export > Audio options window, select
mp3 as the
File Type, and
Entire Mix from the
Source Category options.
If you select
Busses as the
Source Category, SONAR will create a separate audio file from the output of each and every bus. Which is not what you're after by the sound of it.