Trains regularly (every hour) go from Cambridge Train Station to London King's Cross. A direct train will take about 45 minutes. Trains which stop at stations along the way can take up to twice as long. A day return typically costs about £20.
You can also get to London by Coach from Drummer Street Bus Station. Coaches are less frequent than trains, can take up to 3 hours but are usually cheaper.
For more information on London, including hotel booking facilities and theatre ticket booking facilities check out London Online.