Go to Youtube or look at some of the video tutorials they give clues.
You can highlight the clip in the time line and with the mouse grab the volume line at the ends and you can lower the ends to fade in (at the begining) or fade out (at the end).
In Movie Edit Pro you got a pretty comfortable tool to manipulate the audio volume just in a specific place, giving you full control over the audio level and enabling you to implement very smooth audio level changes. It's a special mouse mode - the "drawing mouse" or "curve" mode. Select it from the mouse mode selection in the tool bar or by pressing the shortcut ALT+3 on your keyboard. Now you can basically just "draw" a volume curve onto the audio track by holding down your left mouse button and moving the mouse. You need to be in timeline mode, of course.