First, make sure you have an UNASSIGNED knight in your Knight’s Hall
(ideally one specialized in combat).

Go to the map and hold your mouse over the wilderness areas to see their level and their resource boost.
You can scroll the map by clicking and dragging.
Choose a level ONE wilderness (no higher) and click on it.

Do not attack a Barbarian Camp yet (they are 10x stronger).

Choose attack and then assign your troops and a knight
(you left one unassigned right?).

Select “March” and wait for the troops to arrive.