I'd suggest that when you have an idea, pitch it to your friends.
Not only will that help you form up the idea by actually describing it, but some input at this stage will do you good.
If they think it sounds cool then get to it, if they don't think hard about whether you really have a good idea.
(note that your friends may also not see something great when they have it put to them!)
Its easy to get too inside your own ideas, marvelling at their brilliance when to interest others you need a short and interresting hook that will inspire them. It s like novels, you have to hook them by the end of the first paragraph, otherwise they'll never get to the end and see how good it is.
I'd also suggest that having an idea you want to turn into a game so you can play it is really the first step.
If you don't love your own idea, and want to play your own game,
you can't really expect anyone else to.
Also, when you sit down to write it, really think about whether it is something new.
Is your fantasy game another D&D clone, or does it make more sense as a setting for an already produced system?
To figure this out you have to follow the other best advice, play a lot of games.
There are some games you see people advertising that they seem to think are completly new but only really serve to show they've only ever played D&D.