Cashback credit cards pay you back a proportion of what you spend each time you use them. Therefore set up a Direct Debit to pay it off in full each month so there's no interest cost and you can earn. Just use the card for all your normal spending and you'll make money.
How much? The top cashback cards pay up to 4% cashback for a short period and then a standard 1%. So if you spend £10,000 a year you stand to make £200, just by paying with a different piece of plastic.