Attacking
The appearance of the hostile creature has caused the game to enter combat mode. You can see the Combat Order Ticker on the left side of the screen, indicating that your turn is first, followed by the Orc.
Combat moves forward with each creature taking turns one at a time, in the order shown. Creatures have 100 Action Points each turn. Moving, attacking, and most other actions you can take consume action points.
Now, attack the Orc by left clicking on it or right clicking and selecting "Attack".