As Charny said you need to get to Custom House.
I would use http://www.thetrainline.com
to find out what train you need to get to London. I'd book as soon as you can so that you can get advance fares (which are much cheaper).
As for the tube, it's pretty easy to get to Custom House from Waterloo (as Charny said). If you get into a different station then I have found this website (http://www.tubeplanner.com/planner
) very helpful for finding out which lines to get, where to change, etc. It shows you how many stops until you need to change, how long it will take and which fare zones you need.
You can always ask at ticket offices/information desks if you need to (or you may see some cosplayers at some point who I'm sure will help if you ask).