I have been using the Tube almost since I arrived in London. Its simple, figure out where you are, then where you want to go; not so simple. The Tube is HUGE, lots of lines and lots of stops. Add to that the DLR and the train, you have a mess. In some cases there are multiple routes to the same place; some with no changes, others with two or three changes; sometimes you want the shortest route, other times you just want to sit for the longest period of time. It is comforting to know that even native Londoners have the same problem; I would usually find a couple of other people staring blankly at the tube system map trying to figure out where to go. Another thing, the Tube is HOT! One friend I met carries an extra shirt because he always sweats so bad on then tube he NEEDS to change when he gets to work.
Switching between lines is always a gamble because sometimes the distance between lines through the rabbit warren of tunnels could be as long as the walk to where you were trying to go.