Why does adding a subsurf modifier cause this large flat face to deform? I'm very new to blender so this might be an easy fix, but I have a little problem with the model of my guitar. Here's what I'm working with right now:

But when I add the subdivision surface modifier, the tops go crazy, the big flat faces are not completely flat any more, and there's something wrong with the top and bottom of that carved out part... I would also like to make the edge of that less sharp but I haven't found a good way yet.

For reference, here's a picture of the guitar I'm trying to recreate:

What would you suggest to improve my model? Thanks for the help!
Thanks everyone! I now know that i should stick to four points as much as possible and I fixed it! I tried to make a grid on the huge faces and I got rid of as many triangles as possible and I'm very happy with the result :)





Now onto the rest of the guitar!