Magic arrives...

I'm also a newbie here, but I've worked on two of my own games. One was a flop, and one is in progress and going quite well.

The reason why my first game was a flop was that I started off too broadly: I had a theme, a set of mechanisms I wanted in the game, and an idea of how players win. The result of this wide spectrum focus is that everything I developed was developed with a compromise to everything else and so every aspect suffered. This might just be the result of a new designer, but my new game has been going way better because I think I did something different.

With the new game, I started with a theme and this was to be uncompromised. Everything I added to the game through continuing development must add to the theme or at least fit into it. Having this cornerstone has allowed me to filter ideas and shape the ones I keep to fit.

Here's an analogy:
Imagine designing a game were like compiling a collection of some sort. If you decide from the start that you are going to collect bottle caps, then as you add items to your collection it is going to be more complete and more consistent as you add various items. If the only thing tying the stuff you are adding to your collection together is that they are 'items you like', then your collection won't really appear to be much of a collection at all but rather just a bunch of incongruous stuff.

In short, I would identify what makes a game good in your opinion, and then identify what that is (whether a theme, a mechanism, a kind of interaction) and then focus on designing an instantiation of that and go from there. Figure out what is most important to you :)

As far was where to start in terms of making something, I would start with a game design document wherein you layout your ideas. Decide on what kind of goal players will have (i.e. victory point collection). Categorize your ideas into like groups. See if, on the face of it, any of your ideas don't really seem compatible. If they do seem compatible, find the most important idea and think of a way or 2 of implementing it into game play. Then try to incorporate your other ideas.