There are a few key concepts to perspective sketching.
1. The closer something is, the bigger that it is.
2. That goes for the space between objects too. If objects are separated by the same distance, the further away they are, the closer their spacing.
3. The lines that form edges of objects may converge to vanishing points. In a one-point perspective, lines perpendicular to the view direction will converge.
4. Most vanishing points occur on the horizon (notice that the red dots are on a horizontal line in the image above). The horizon is where the sky meets the land (if the land is flat) or water.
5. The horizon is your eye level.