This is the basic positioning of your eyes, just like pointing a camera. What's the best Line of sight, the straight line between you, the Observer, and the Object or focal point of the scene before you?

Study and consider what you're looking at. Is this the best viewpoint to find the best composition? Where are you physically in relation to the scene you are considering painting? Are you elevated, looking downward at your subject? Are you looking up? Standing, sitting, or kneeling? Flying?

Once you know where you are find the horizon.

See where the sky meets the land? That's the horizon line. It's on the horizon. If objects are blocking your view of the horizon line, turn around until you can determine where it is in relation to your line of sight and extend that knowledge into your chosen view.

The vanishing point is a point in the far distance at which your eyes can no longer see. Since the farthest we can see here is to the earth's horizon that's where most vanishing points live. Perspective lines start at the vanishing point.