The basics of perspective

I'll try to explain the basics of perspective in a simple way.

To get perspective in an image you'll need 3 things:
1. Horizon.
Doesn't mather if it's in or out of your image. In this tutorial we'll put it inside the image, this way it's easier to explain.
The horizon is always at the same hight as your eyes are.
Remember at the beach: standing up the horizon is high, sitting down it's low.

2. Vanishing point(s).
A vanishing point is a point on which all parallel lines coming from an object come together. These lines are called convergence lines at they are #3 we need.

3. Convergence lines.

Because this is a basic tutorial we'll use a cube placed flat at the ground.

As you can see we have 4 things:
1. The cube.
2. A flat (shiny) surface.
3. The horizon.
4. A sky.

I'll draw all parallel lines (the convergence lines) coming from the cube. Each set get's a different colour.
You'll see the green lines coming together at 1 point at the horizon, that's the vanishing point. This also works for the blue lines, they'll also (eventually) will come together at the horizon in a vanishing point.

OK, how about the yellow lines you'll ask.
They'll also will come together in a vanishing point. This vanish point will be high up.

Now we'll get more cubes in, they all are parallel to eachother. You'll see the convergence lines will use the same vanishing points.

You'll see the black lines coming from the cubes, yellow from the shadows and the red from the reflections.

These are just the basics of perspective. You can imagine other objects in your image not being parallel to eachother (they'll have different vanishing points) or objects which are tilted, but no mather how you place them the basics are always the same.
There will always be vanishing points , convergence lines and a horizon to be taken in account.