The Hololens fits over your head like a topless helmet. Proper positioning will enhance the mixed reality experience. Follow the instructions in the Microsoft Fit your Hololens guide.
Calibration and Sensor Tuning are important processes that improve the Hololens environment. Calibration aligns the device to the user's line of sight. Sensor Tuning ensures that the device is capable of properly interpreting the spaces where the device is used. The processes for calibrating and updating the sensors can be found here.
Your eyes are your cursor. Sounds pretty funny but in the Hololens world the object you look at becomes the active object. For example, if you look at a button on a window, you will see it brighten and/or display an outline. A circular object may also be visible. Find more about Gaze.
Hololens uses a variety of methods to interact with applications. Gestures are a set of hand motions that control objects. Microsoft provides guidance on hand gestures at this site.
Holographic objects and the device itself can be controlled using voice commands. Cortana is actively listening in the background for the words 'Hey, Cortana'. For more voice commands, check out this website.
The Hololens includes a handheld clicker which can make it easier to control objects without raising hands. An explanation of how to use the clicker is provided on this site.