Before riding your bike you should check that it is safe to ride. If it is too big or the handlebars are in the wrong position, you won't be able to control it properly. You should also regularly check your bike to make sure it is in good working order.
Government website Directgov provides information on bike checks including tyres, brakes, handlebars, steering, saddle, chains, gears and pedals.
See Directgov's advice on getting your bike ready to ride at: http://www.direct.gov.uk/en/TravelAndTransport/Cycling/DG_190030
Although it's vital to make sure that your bike is in good working order, it's just as important that you're ready to ride. You'll need the right clothing and equipment if you're going to cycle on busy roads.
DirectGov provide useful advice on helmets, clothing, lights and reflectors, how to cycle safely on the road and the Highway Code.
See Directgov's cycling safely advice: www.direct.gov.uk