Unlike some other games in this genre, once you have placed troops in the enemy base, you cannot give them orders over what to attack. One of the most important things to learn, therefore, is what each units' preferred target is.

Barbarian:
They will attack the closest structure to them.

Archer:
They will attack the closest structure to them.

Goblins:
They focus on attacking buildings that will get you resources.

Giant:
They target enemy defenses as a priority.

Wall Breaker:
They will attack walls before anything else.

Balloon:
They are specialized for targeting enemy defensive structures.

Wizard:
They have no preferred target and will attack anything.

Healer:
They have no attack function.

Dragon:
They have no preferred target and will attack the closest structure to them.

P.E.K.K.A.:
They have no preferred target and will attack anything.