I have drawn the base plane of a guitar and I want to use it to make the body
Select your sketch and then use the extrude feature. There is an option for thin feature. When you select this you can choose how thick you want your solid to be and what side of the line to be on. I can try to put a tutorial together later.
Here is a quick tutorial for you.
Answered with a tutorial: https://grabcad.com/tutorials/how-can-i-make-a-thin-surface-out-of-a-closed-spline
you can also use surface extrude>thicken, planer surface>thicken>shell etc lots of ways, Stephan's way is the shortest and easiest!