While VOIP traffic itself is not more expensive than regular telephone traffic, sometimes new equipment will be needed depending on what you need to do.  A good telephony consultant can guide you through this process.

This depends on what you want to do.

With an internet connection,  and a free SIP account from a provider such as Gizmo, you can start using VOIP.

You could add an Analog Telephony Adapter (ATA) to that and use your regular analog telephone with Gizmo.

A better way to get some more advanced knowledge might be to download and install Asterisk, a powerful open source PBX, which supports, among other things, VOIP.

If you are running a company, large or small, and already have an installed telephony solution, there are still many ways that VOIP might be appropriate for your organisation and can be integrated with your current solution.  There is no need to purchase all new equipment just to use VOIP.