You can attack zombie herds (NPC) by clicking on the map or other players via the button next to the map.

On the map, choose the light marked area with the crossed machetes and choose one square occupied by zombies. Click on “Start attack” button to get to the menu to set up for battle. 

Your available positions are the red ones. On every spot you can position one of your units, choosing any type of unit you want to use first (providing you have unlocked several). If you would like to use more than one unit type, make sure that you do not position all your units on one spot when starting to arrange the fight.

As soon as your line-up is ready, click “fight” to start the battle.