Preventing early elimination from ruining someone's first game of Diplomacy?
Like Ry4an said, it is pretty much guaranteed that some people will be eliminated (or nearly so) early on.  You'll want to make people aware of this at the beginning, and set some expectations going in of how people will react (ie Is it OK to just give standing orders for 1 or 2 units and leave? Is it OK to proxy control of units to another player and leave?  Are eliminated players allowed to listen in on negotiations between other players?)  You might want to have other games on hand so that eliminated players can entertain themselves during the negotiation phase and check in on the Diplomacy game occasionally if they're interested in the final outcome.  

Another option that I mentioned here is to plan to play several mini-games with pre-determined ending years.  You won't get the full end-game experience, but several shorter games with some time to debrief in between might be a better first Diplomacy experience than one full game with a number of eliminations.