You can do that easily with edge loops.
Start with a circle.
Extrude it (Press E and Scale it S).
Take the outermost vertices and grab them (G) create the shape of the guitar.
Until you have the full outline of your guitar.
Create edge loops by pressing CtrlR. And dragging the mouse to the middle of the geometry.
You can choose how many edge loops you want to create.
Keep adding edge loops to refine your shape.
Another way to do this is modeling with curves:
Add a Curve_Circle object.
In edit mode subdivide it and add an outer circle. Subdivide it and shape it to the form you need. (It's very important that your circles have the same number of control points, 8 in this example).
Make sure the fill option in your curve is set to none:
Exit edit mode and convert your curve to mesh by pressing AltC and select Mesh form Curve/Meta.
Then in edit mode select all your vertices and do bridge edge loops (press the Space Bar and type Bridge...)
Then you can do edge loop cuts with CTRLR
Note the adjustments you can do changing the factor