We would like to recommend you a very good travel agency specialized in world trip flight tickets. It is Zip-World.fr: http://www.zip-world.fr/
They have different categories of flight tickets: Zip Cheap, Zip Cool etc or Zip 4 u where you can design your trip with a unique price for you.
They offer an interactive website with a visual map and and more detail information of your itinerary. You can contact your ZipWorld agent at any time through an account designed for you once you have subscribed. Their conditions are very attractive as you can change your travel dates for free with many companies. Some companies will ask you some extra fees.
Another site we used to compare was http://www.bootsnall.com/ This site is more like DIY and has a lot of information on RTW tickets, so you get an idea of what it may cost.
To start off; have a clear idea of places you want to visit and estimated dates you will be in each country, you can then start to design your world trip!