In order of importance from first to last.
Cottages house slaves and every level increases the amount of slaves you have. I prioritize upgrading cottages because it's better to have slaves hanging around the resource producers than not having them at all. I'd rather have 15 extra slaves working in my stone quarry and move them to food when I can.
Resource buildings should be prioritized second. Now that you have slaves waiting and producing other resources you can focus on the resources you actually need.
Once you've got all the buildings you prioritized to the highest level the can be you should upgrade the town hall. Usually I leave Resource buildings one level below town hall because I don't really need them. I try to only upgrade them when I really need them. But if not timed correctly you may miss out on resources.
Town Hall is obviously very important, but it's basic purpose is to increase the level cap on your buildings. Otherwise it doesn't serve any other purpose. So for this reason I like to avoid having to upgrade it until I really need to.
You're also gonna need the barracks and research buildings.
Acadamy and Arms Center These buildings provide very important research for your cities. Once you have researched a technology it affects BOTH cities and thus doubles the bonus if you have a second.
Barracks I know the barracks don't directly contribute to your resource gathering but it is REALLY important that you upgrade this. Plundering is a BIG part of this game and it's important to be attacking other players for resources as well as generating your own.
You should be trying to maximize the amount of troops you can make before you hit level 15. You really don't want to have a low power level when you're that level or your enemies will start targeting you for bloodbaths. If you have a high enough power level they'll probably refrain from that.
Attack other players.
A lot of players seem to really be focused on building their city rather than getting out and attacking other players. What they don't realize is that it is possible to farm many inactive players without losing any soldiers. Some of these inactive players are still generating lots of resources that are just accumulating. Sending enough units to win a battle without losing any troops may mean you walk home with hundreds of thousands of resources.
The problem is you won't know who is really inactive or not unless you attack them or message them. I know many of us aren't down to start a war but if you do attack an active player I'm sure you'll get a message asking you to stop. Although it may be a mean one. You can simply apologize and say that you thought he was inactive and that you'll stop. In one case I even sent back the resources that I earned so we really had no hard feelings.
Don't be afraid to get out of your shell and attack other players in other regions too! You can even go so far as to monitor their progress if you're really feeling paranoid. I like to look to see if their alliance is really powerful. If it is then I tend to stay away, If not I'll go right in.
Keep on attacking and you may hit a jack pot from time to time. This really gives a boost to your economy for the most part. It just takes time to tap all of the commands. It's a good way to occupy time while waiting for something or during class if you don't feel like paying attention.