Take the District (green) line in the direction of Tower Hill/Upminster - those are the termination points of the line, based on which train it is. Either one is fine.
Get off the train when you get to Embankment Station. Transfer, within the station following the signs, to the Bakerloo (brown) line in the direction of Elephant and Castle
Get off two stops later at Lambeth North and find the "way out" and walk south-ish to get to the IWM, which is on Lambeth and St. Georges Rd.
If you want a more interesting trip, you can take the District (green) line and get out at Embankment. At that point, walk over to the Waterloo Bridge, go across it, and follow the road down to the IWM.