This is a fairly broad question. At the simple level, you will need some type of router, which you probably already have for internet access, but you may need one with a built-in switch if you plan to have more than one device connected to the Internet, like the VoIP device and your PC. Next you will need a VoIP gateway or IP phone. The type and features you will need will depend on what you want to use VoIP for, what application are you looking to do? Once you know this, there may be other types of equipment you will need. Finally, you will need someone to call to or receive calls from. Again, this depends on your application.
Mike