See an accountant.
It really needn't be complicated at all. A company can be set up and registered for less than £100-£150. (Can't remember exactly)
Why do you want to set up a LTD company though? From the advice of an accountant or just because?
Apart from the cost of accountancy fees being higher because you will need to file a full set of accounts each year, there really are no other extra charges compared to being a sole trader.