Who moves first- and when?

How does the game decide who's move is made first, after a turn ends? It's so extremly frustrating sometimes. In some games, it's me who always moves first and then it's really easy to win. But in other games, my enemies seem to have the "luck" on their side.
Since moving first is even more important than any strategy or tactic, I would like to know how it works.

I thought it's based on who set's his moves first, but I have my doubts concerning that.

Explained here:
http://afterwind.com/forum/topic.php?topic_id=29