DBP can support over 250 connections in a peer to peer multi-player environment. So, an online game is possible (a MMORPG is a totally different animal.). Design wise, you probable have two major programming jobs to do. One is to write a DBP client program that actually plays and participates in the game world. The second is to write a dedicated server program that processes all the player inputs and sends out the results. The more people you have connecting, the more horse power the server needs. Eventually, it will get to the point where you won't even want to run graphics on the server machine.
Programs in general are lots of small solutions to problems.
As CattleRustler said, start small, learn the language, get a feel for solving problems in bite sized chunks and then move on to the deep programming and design stuff.
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TAZ