To open a new sawmill, click on the icon with a factory on the bottom left of your screen. Usually, you need some amount of money and reputation points.
 
New sawmillBuild a new sawmill
 
Reputation points are the in-game currency. To start earning them, you need to build the Workshop. You can find it to the right of your sawmill.
 
Build the workshop
 
You can get reputation points by completing workshop orders for the construction of houses.
 
Reputation points