The easiest way to understand the economies of Widelands tribes is to play the in-game-tutorials, which show up all the important things needed to play.
For those people that don't want to spend time playing our great tutorials X-D, here are a few examples of how you COULD start:
This offensive strategy is much harder. It can bring a fast success, but also might end in a total destruction of your tribe/empire, while you will have to start work with on demand resources, and won't have enough materials for a quick rebuild after an too fast attack of an enemy, at the beginning.
The idea of this strategy is to take everything you, get. Land gives might and saves resources, this will be the leading line:
You start with building four lumberjacks, two rangers/foresters and a woodhardener/sawmill.
As soon as most of these buildings are complete, you begin your fast expansion to every side from your headquarters. For this you use sentrys (and barracks if you play Empire)
In the following time you will expand further, as soon as you occupied new land. Only the militarian buildings at frontier will have full occupation, all others inside your land will stay with only one soldier.
Only on important points (like mountain-passages) you build up bigger militarian buildings.
During this chapter, you try to build up a base economy: Some further lumberjacks and rangers/foresters and woodhardeners/sawmills, as well as quarries, stonegrinders/stonemasons and Ferneries
If you feel like you get enough buildingmaterials, you also can start to build some food-economy, to fill your stock with food for future mines.
End of the expansion-chapter will be, as soon as you have only 1-3 soldiers as reserve left.
After that first chapter you build a strong food-economy, search for ressources and build a few mines.
If everything went well and your economy works fine, you than can start building up all the remaining buildings, like burners, smelters, smiths, trainingscamps, and bigger militarian buildings. And of course you can start to expand further.
Now it'll be just a matter of time, since you are strong enough, to attack and destroy your enemies.