ZeG, finish some maps, get a portfolio. Even without mod experience you'll have a much better chance at getting accepted. Additionally, get some contacts within mod teams and show them what you're working on. It's easier to get invited by a friend.
But yeah, knowing what you want to do or what you want to map... that's important to find out. Invest some time into that, as it's obviously what's keeping you back. Think about what you like, try to channel that to a map.
What I often do is keeping idea's for later. I often can't work out something at the moment I think up on it, or I fail or never finish it. It seems 80% of what you do never gets anywhere. Still, I learn from it, get experience through it and later on, it provides me with idea's I can work out at a time I look back at them.
Same goes for games: if you know what you want, you'll find a way eventually. I know I want to create games and I also know what kind of games. That makes the job easier, as I can focus better on things I need to learn and if gives you a more solid base to work on in general.
Programming is just like that. I'm programming for about, say, 3 years now. But the first 1.5 years I didn't do much simply because I didn't really know what to do with it. Now, I'm getting more inspiration, I'm getting idea's and things I want to work out that challenge me. Makes me program much more and I like it.