Carbon Independent has an easy questionnaire to get an idea of the CO2 and other greenhouse gas emissions that you are responsible for, as an individual, over a 12 month period.
In section 1, you enter household data, so that items such as household heating and car use are shared between the members of your household.
In section 2, you enter personal lifestyle and travel choices that apply to you as an individual.
The calculator then gives you a chart that you can print out
And the Muswell Hill Sustainability Group has some other suggestions here.
Carbon Footprint says that “Carbon offsetting projects help to combat global climate change as well as caring for local communities. In many instances providing much needed employment, health improvement, biodiversity, reforestation and broad social benefits to impoverished communities.”