Ebico do a handy online CO2 calculator which takes your energy usage details and has a guess at how much CO2 you produce - in my case, it's about 6 tonnes per year. They then go on to suggest that you buy CO2 credits to cover it. I'm not entirely convinced by that - the theory is, because you are buying the credits from the general industrial trading scheme, you are capping the amount that industry as a whole is allowed to produce. But the caps are fairly generous, and people wouldn't be selling them if they needed to produce that amount of pollution. And if a company starts polluting more, who's going to stop them - or measure it precisely enough to prove it ?
I would have thought it would be far better to spend the money on renewables schemes, or planting trees, or sending Goats to Chili, or on a few low energy lightbulbs - otherwise you're just lining the pockets of some market trader with no net practical outcome. In particular, a quick google for 'carbon scheme' came up with a stack of articles entitled 'carbon scheme doomed' and similar.