The main challenge is to have a steady income of resources. You need to maximise the resources you gain every hour.
One way to do that is to acquire valleys. Each valley (grassland, forest, dessert, hills, lake) gives a percentage resource bonus. Hover your mouse over a valley it will show you the following information:
1. Type of valley it is
2. What type of resource increase it gives.
3. Which type of resource production it increases.
4. Percentage by which it increases production of resource.
5. If it is currently owned by any lord.
Every valley has troops hidden within them. To conquer a valley you have to attack a valley and kill all the hostile troops. Once you have attacked, if your valley quota is not full then the valley will come under your ownership and it will continue to give you percentage bonus of your base resource production.
The higher the level of the valley, the higher resource bonus it gives but the higher the level of the valley the more hostile troops there will be, so to conquer a higher level valley is more difficult.
The number of valleys you can own is equal to the level of your town hall. If your valley quota for a particular city is full and you want to acquire a new higher level valley then you will need to abandon one of your low level valley and free up one valley spot.
As a guide - to conquer a valley scout the valley first, then send 3 times the amount of troops that are found in the valley.
Be sure to check if it is owned by another lord or not. If it is a free valley then send about 100 scouts. If the level of your informatics is low then you may not get the exact amount of troops in the valley. Your will see they are denoted as Pack, Lots, etc.
To abandon an existing valley go to "overview", then "valleys" . "View" the valley that you want to abandon and click "abandon".
If you conquer the valley, your army will camp there and while camping the troops consume twice the regular amount of food, so you will see a sudden increase in "troop upkeep". So recall your army from the valley. To recall your army from valley.