Once at the airport, whatever that airport is, you always have several options available to you in order to reach Central London.
From Heathrow, you can both ride the tube, a coach or a, definitely more expensive, taxi.
From Stansted Airport you can either ride a train, i.e. the Stansted Express, but note that it will terminate at Liverpool St. Station, a taxi or a coach.
Should you opt to use a coach, you can both reach Central London by the National Express Services (Single ticket £10, return ticket £17) or a Terravision service (Single £9, return ticket 14£). As you can see there is a small difference in price between the two companies, but you’d better decide which service use according to your final destination in Central London.