Switching from claims to classic is possible (as @saumilm describes), but it's not supported and you have to recreate all permissions.
In your comment to @saumilm you mention that you haven't created your webapp yet, wants to start with windows, but have to option to add forms later. In that case you're free to choose, forms is claims, so you can start with classic and switch to claims (which is fully supported) or start of with claims and stay there.
One thing to note is that a few BI features isn't working in claims, which means that on one hand it might be good to start in classic where everything works, on the other hand if you want to be sure you can switch to claims without any problems, then maybe you should start there.
Note that in SP2013 Claims is the default and Classic can't be created from the UI.