Freeview comes down your aerial and is free, but only has a limited number of channels. As your TV already has freeview you can use that for free.
Rather confusingly there are TWO services called Freesat.
One, provided by Sky, the other by the BBC/ITV.
BOTH come in via your existing "dish".
The freesat from Sky uses your existing Sky box and offers around 200 channels and no monthly subscription. You do need a Sky card (only �20) and you can read more here
http://www.freesatfromsky.co.uk/
The freesat from BBC/ITV needs a new "box" but no monthly charge, and offers about 140 channels, some in HD if you buy a new HD box.
You can read more here
http://www.freesat.co.uk/
As you already have the Sky dish and Sky box then it makes sense to go for the Sky freesat offering (unless you want to watch some channels in HD then go for the BBC/ITV offering).