In addition to what blah said about experience (trust him, he is da network man), when you are starting out that is what you lack. But don't let it fret you.
Since you seem to have the other stuff down solid, you could take a time away and dive into networking.
From my humble experience, supporting networking in your game will also mold your game into a certain cast.
You probably won't be able to do stuff you can with just a single player game.
Then make some prototypes that match the core concepts of your game.
Be prepared to throw them away again.
In then end you might be able to work the networking into your game with only little effort (if so, then the force is strong with you
Or you might not and have to rewrite your game. (which is perfectly ok, we all have been there... far to often)
No matter what, adding in networking is a highly valuable experience that will only make you a better game developer.
In closing (and a little OT) blah is also painfully correct when he mentions the network programming being pushed off to later.
Sadly this seems to be the norm (even more in the 'professional' field) and in case of anything non GFX related even worse (AI, content f.i.).
Today the first thing you will see from a new game is the Bling. And then when the development reaches the end they remember they also have to add in a game somewhere.
Even stranger because by the time they actually reach the end of the development and have finally added in the useless 'game' feature no one wants anyway, they discover that their GFX are outdated.
So, like blah is saying, do not fall for the 'we'll do it later' trap. It could really come back to haunt you.
The good can compensate, others fail.