Players can be eliminated in two ways. 1) They get "wiped out," losing all of their supply centers and pieces. 2) They are surrounded and about to get wiped out, so they resign by declaring "civil disorder." Their piece remain on the board until eliminated, and they can't move, but other players can "support" individual pieces located in cities to prevent them from being captured by a third party.
Elimination usually takes time (at least two or three game years), but is part of the game, so people need to learn this. If everyone is new, there may be a lot of back and forth that will prevent people from being eliminated quickly. And even after elimination, there is an incentive for people to stick around: 1) to see others get eliminated and 2) to learn from the experience of others.
One way to greatly reduce the chances of your new players' being eliminated early is to have them draw separately for the corner countries that are less likely to get eliminated. http://boardgames.stackexchange.com/questions/3796/in-diplomacy-should-you-have-separate-country-draws-for-players-of-differentIn Diplomacy, Should You Have Separate Country "Draws" For Players of Different Experience?
Once these players have drawn their countries, you can have a full draw for everyone else.