Resources can be harvested by the player by moving close to the resource, looking at it and holding E until the circle indicator is full then the player will receive an amount of resources.
To change the amount of resource the player harvests you can go to the PlayerBuildingComponent and in the class defaults search for the Resource Collection Amount this in the number of resource that will be collected from a resource.
To change the amount of resources a resource has you can navigate to its blueprint the default resource blueprints are the RockResource and the TreeResource once you are in the resource blueprint search for the Resource variable and click the drop down option, it should look like the image below.
You can then click the drop down on wood or stone and set the Resource Cost to the amount of resources you want this resource to contain. For example in the image below shows a resource that would have 50 wood and 0 stone.
To add a resource to your level simply drag the resource blueprint from the content browser in to your level.