How do I boot from my Mac partition? I tried to install Windows 7 on my Mac via Boot Camp, but the disk of Windows 7 doesn't work. When I tried to install the Windows 7 it shows a black screen.

Now I want to boot back to OS X, but I don't know how.

Do I have to re-insert the Mac disk? If yes, how can I take out the disk from the Mac?
Hold the option key when you power on and you can choose the working mac partition or insert a bootable DVD to get back to disk utility or a working os. This is called the Startup Manager screen. 

The eject key will work once the Startup Manager is running so you can eject the windows DVD. 

As I didn't explain the option key - here is a great article explaining how Mac users refer to the keys on the keyboard as well as some handy shortcuts for later once things work again.