Before you start to think about getting other people involved there are several key choices to make:
What game engine are you going to use, or will you code one from scratch?
What will attract people to your game?
Who is your target audience?
I try to keep things open source, so as a game engine I chose to use Blender's
internal game engine. Other free options are Unity
and many others. Have a look at the wikipedia page
for game engines.
You choice of engine may be influenced by things like:
Programming languages you know
Licencing of published games
Game-play type. (ie Spring is for RTS only)
When you create a game you have to think about who will play it and how they will find out about it. As I found out, making a game is 10% developing and 90% promoting.
Most game projects have a website, for consumers and developers.
Publish WIP versions of your game on the web.
Make posters and distribute them on the web.
You also have to think about who will play it. What makes it stand out from other computer games? There are millions of FPS's and RTS's, yours must be slightly different to make an impact. But it can't be so radical so as to turn people away.
Make sure you have an audience.