Auto-attacking is, in essence, the simplest way of dealing damage and thus the most basic method of applying pressure. An auto-attack is performed by using your weapon's default attack without using any attack skills and is executed by first selecting a target then pressing the "Action: Attack/Interact (Do It)" key (default: Spacebar).
Note that, while on the tutorial areas of each campaign, on excessive use of the "Action: Attack/Interact (Do it)" key, a useful tip in the form of a dialogue box will be shown (see right).