Imperial College is located in South Kensington, a very busy area of London. This coupled with a lack of car parking facilities means that coming in by car, unless you arrange a parking space in advance, is difficult.
The London Underground is probably the most convenient way. Travel to South Kensington station, which is on the Piccadilly, Circle and District lines. Inside the station and to the right of the ticket barriers as you emerge is an underground walkway that leads to both the college and the nearby museums.