I see that I'm 4th on the "most improved for december" leaderboard and that I am in line for 250 credits.... Awesome!!! what can I do with these "credits?"
Credits can be used to purchase Premium Membership or entry in the the Conquer Cup. However, since both of those cost more than 250 credits you would need to acquire more to buy anything with them. You could do that by placing on the monthly prize boards again, winning Auto Tournaments, completing surveys & stuff, or simply buying some.
Actually, I don't think you'll be able to anything credits for being on the monthly leaderboard for December ... at least not yet. But good luck staying up there for the rest of the month.