These instructions refer to Gnome but they should work under KDE or fvwm as well.
You can find common applications under the Applications menu. Go to the application you want and left click on it. Sometimes this will result in a pop up message as well as executing the application. The pop up message may be useful but typically these messages are more useful when running the application from the command line. You can add short cuts on the desktop or top panel to applications that you use often by following the instructions here Adding shortcuts for common applications.
You can also use the tap command to run Math applications from the command line.