Cambridge is small enough that walking and cycling are normally the best ways to get around. There's very little parking so driving isn't recommended, but the taxis are good.
The "Uni 4" bus is also very useful - it goes past the William Gates Building, all the recommended colleges and hotels, and near to the railway station.
Arriving by plane
Stansted Airport is the closest major airport, a ~40 minute train journey away from Cambridge by train.
From other airports the best options is to go to London and take the train from King's Cross.
Arriving by train
Cambridge Railway Station
is around an hour from London King's Cross or Liverpool Street, or 40 minutes from Stansted Airport. From the station it is best to take a taxi or the Uni 4 bus.
Arriving by car
Madingley Road Park and Ride carpark is just over the road from the William Gates Building.