Assuming you mean freesat from Sky (which uses your existing Sky dish and box) then see this web site.
http://www.freesatfromsky.co.uk/
The FAQ says
Q: I already have Sky digital satellite equipment - what else do I need?
A: All you need to gain access to the free channels available on the digital satellite platform is a Freesat from Sky viewing card, which costs just �19.57. If you�d like to access the interactive services available, you�ll also need a working phone line. To purchase a viewing card please phone Sky on 08448 244 400.