Dear Lisalou;
The main options for home users are:
1) A Router with a VoIP port > Then you can set up the voip service on the router > and you can use whatever (old) analogue phone on that device for VoIP calling.
2) Use an ATA (Analogue telephone adaptor) > It's does the same as the router > But so you can avoid buying a new router (An ATA is cheaper), then you connect the ata to a free port on the router ..and the phone to the ATA.
3) Buy a Voip Phone, also to be connected to the Router. Many possible devices herefor.. With fixed Cable's Or Wireless (Dect VoIP)
For Wireless at this moment the Siemens gigasets (like c475 ip or S685 IP) Are very popular.
Also offcourse you will need an VoIP account to put on your VoIP device...Search for 'VoIP providers' to find one..
A good site to find Info about VoIP devices[ATA's / Routers / Phone's ...] (And where to buy them cheap

) is voip-info.org.
Hoping this 'Basic' info helps you a bit