Search the help files for “Clipping Mask”.
If the guitar is a raster image, you will have to trace its outline, either with the Pen Tool or with the Live Trace Tool, the former being superior, thus creating an outline path. This may be used as Top Object if you Object>Envelope Distort>Make with Top Object. Or as an Object>Clipping Mask. I understand your description to indicate that the former is what you need.
Make a copy of the shape you want to use and place it over the text you want to manipulate. Be sure the shape is above the text. Select both text and shape, choose Envelope Distort from the Object menu, then choose Make With Top Object. Voilà! The text is distorted to fill the shape.