You need a new LNB on your dish, you need two cables from the dish to the Sky+ box (you currently only have one), and you need a new Sky viewing card.
Not easy to do yourself me thinks.
I think the �10 a month for Sky+ has been removed (or is being removed).
The only extra cost is �99 for a Sky+ box which is a one off payment.
Phone up Sky and say you want Sky+ and I think the only cost will be the �99 (and maybe a fitting fee).
Once you have Sky+ you will never regret it, it is pretty impressive.