Otherwise, I would say go to a travel agent in your home town. (I wouldn't recommend trying to do it online, unless you're very sure of your itinerary, and cost isn't an issue.)
Have a look in the travel section of your Sunday paper there are loads of travel agents that offer round the world trips. I would also recommend trailfinders, but there's also STA, Quest Worldwide, Travelbag [who I think are now part of ebookers] and plenty more