You can begin with Cyclops Lane first, then Triton Avenue and finally Firecat Alley. Interestingly enough, I just listed these in order of increasing difficulty.
Colossus Boulevard will become available after completing these areas. It's a side quest and does not need to be done in order to get to Krocktopia, but given the limited amount of help available, especially when Krock may get difficult, you may want to do this area (Colossus) in order to stay ahead of the game and quests!
It's not an easy feeling being stuck on a quest just because you lack sufficient levels etc to complete it!
You're going to have to do all three alleys in order to reach level 12, and the order doesn't much matter.
Like Star Edward, I'd agree that Fire Cat Alley is the most difficult of the three streets (because the boss at the end is pretty tough). But otherwise, which street challenges you the most will mostly depend on what school you're playing.
Personally, I always Triton Avenue, then Firecat Alley, and then Cyclops Lane. In my opinion, this is the easiest and most convenient way to go. I've always never had trouble battling / dueling using this method, and the Firecat Alley bosses are much easier than they seem / people say they are, especially if you join a server that's crowded / full of other players to work with :) I find all streets equally as easy, and are easily interchangeable. The difficulty does not quite drastically vary between which street you choose.