Why is the mesh deformed when sculpted with Subsurf modifier? I am creating a pillow for my couch.

But when I click on my base, it just deforms it like this: 



Does somebody know how to fix it? 

After ticking dyntopo it looks like that...
You don't want to use a subsurf modifier. If you apply it, it will be incredibly slow or far too low poly. For sculpting, use the multi-res modifier like this:



Otherwise, dyntopo is very easy to use (although the resulting topology is less than desirable). Here is a sculpting example with dyntopo: