Working out your carbon footprint is a measure of the carbon dioxide emissions created by you in your activities over a year. In the UK emissions from transport make up about 20% of total UK emissions - so it's really important to minimise your impact of transport to reduce your carbon footprint.
This graph shows how far you can go, using different modes of transport, to create 1kg of CO2:
Flying is a hideous waste of energy, creating enormous emissions - especially given most flights cover vast distances. One return flight to Australia will use as much energy needed to heat and power your home for six years.
On the other hand, the bicycle, one of the most efficient machines ever invented, allows you to travel reasonable distances, carrying a reasonable weight, yet has the lowest emissions of any mode of transport.
Reducing how much you fly and drive, changing instead to public transport is the most important starting point to reducing your carbon footprint, and hence global climate change. Walking or getting on your bike is the best thing you can do - and there are few excuses for those local journeys!