I recommend you design it from the start to support multiplayer; hacking a singleplayer to add multiplayer is usually a lot more effort than just doing it right from the start.
Multi-player programming isn't too difficult, but it is kind of tricky for a beginner to get it right. It's not just a simple matter of sending commands through the connection; you also have to figure out a method for dealing with latency and disconnects, methods for screening input to try to prevent hacks and exploits, etc...
If you haven't completed a single-player game, though, I recommend you do that first. In the process, think about ways that you can structure your code so that when you do design a multi-player game you know a bit more about how to put it together. Come up with strategies for object management and communication that will facilitate multiplayer.
Also, do some smaller-scale multi-player projects. Simple chat clients, tests to figure out synchronizing objects, etc.. The more time you spend here, the better you will be able to spend your time on the big project.