Once you get to level 25, you can open up one of the regions. When I asked Club members about which region to open, I didn't get a clear answer. So here are some helpful hints.
There are five regions: Green Valley, Cactus Canyon, Limestone Cliffs, Frosty Fjords, and Sunny Isles. The advantages of opening up a region are (1) you get Contest of Mayors (CoM) assignments associated with the regions that you have activated, and usually these assignments aren't difficult, (2) you can get an additional storage with each region, and (3) you get allocated additional Maxis Majors with each expansion.
My advice would be to leave Cactus Canyon and Limestone Cliffs for last, as these do not have beach or mountain special buildings. The beach and mountain expansion CoM assignments are easy and high CoM point value. You can usually get beach or mountain expansion items on Global or from CoM prizes or from Club members. I'd start with Sunny Isles and Frosty Fjords, as this gives you both beach and mountain, respectively.
Another thing I should explain is that each region has its own currency. Sometimes you get a CoM assignment to add, say, 3,000 regional currency points. You get regional currency by upgrading buildings, selling items to the region, or completing truck exports. In order to add residential buildings, you have to provide regional services in addition to fire, police, and health. You can sell back any regional buildings that you build for 1/2 of what they cost to build (bulldoze). So if you have at least 6,000 regional currency, you can build and bulldoze regional buildings to get the 3,000 points of regional currency for the CoM assignment (easy and quick to do).
