In the vast majority of cases you will be able to switch your energy supplier with just a few basic bits of information such as your postcode, how much you spend or how much gas and electricity you use. You should be able to get all of this information from your gas and electricity bills. The more accurate information you can put into the calculator the better the results will be in suggesting the cheapest supplier for you and how much you are likely to save each year by switching.
Once you decide to go ahead you will need to set up a direct debit with the supplier so that they can start the transfer process. In a small number of cases you might be asked for your meter number but normally this can be found on a bill or on the meter itself.