Usually, the horizon line is at your eye level, but this can be a bit hard to see in some situations. There may be hills or trees blocking the true horizon line. Also, standing in a valley or on a tall hill can make things seem a bit different.
Just remember that everything flows to the horizon line. It is the furthest point in the picture.
Now, let's go over the different types of lines.
You will run into three types of lines with drawing perspective.
Vertical lines run perfectly straight up and down
Horizontal lines run perfectly straight side to side
Diagonal lines run at an angle.
Anything that is not perfectly straight is considered a diagonal. This will help clarify the difference between the different types of perspective.
One-Point
With one-point perspective, everything flows to one point on the horizon line. This makes rooms or buildings give a "hallway" effect. On a street, you would be looking straight down a long street. If you were to turn at an angle, you would get two-point perspective.
With one-point, you "must" face walls or objects straight on. Think of walking straight down a hallway.
All horizontal lines and vertical lines are straight with one-point. Diagonal lines point to the vanishing point on the horizon line.
The vanishing point is basically where you are standing. In two-point or three-point, the vanishing point is a little more arbitrary, so don't worry about what it is exactly. Just be able to see where everything is going because that is the important thing with drawing perspective.
Two-point affects the horizontal lines. Now you can see why I made that illustration to talk about the different types of lines. Go back and review it if need be.
As you can see, with one-point, we had a straight wall. Now, both walls are diagonal. Look at the red lines. That's the different in one and two point.
Vertical lines are still perfectly straight up and down.
We now have two vanishing points, so both sides of the object go to a vanishing point now. One side goes to one, the other side goes to the other.
You can also see that the vanishing points are outside of the picture plane. They could also be inside the picture, but most of the time, they are very far apart. This means that you won't see them in your photograph at all usually.
Both the top and bottom edges of the object will point to a vanishing point. The only time you will get a "straight" horizontal line is when that line is exactly on the horizon line. With these drawing perspective examples, I recommend using a ruler to make things more clear when you practice.
Any lines close to the horizon line will not be very steep. However, the further you get from the horizon line, the steeper everything will get. That's why the tops of tall buildings look so steep. Compare the angles on the bottom of the building (which is close to the horizon line) to the angles on the top of the building (which is further away from the horizon line). The top is much steeper.
If all of this is confusing, don't worry. Just practice the illustrations. Drawing perspective is very complicated, but you really just need a basic understanding about what is going, and your drawings will look fine.
Three-Point
As you can see, with three-point our straight vertical lines have become diagonals. Look at the red lines. In fact, all of the perfectly straight lines are gone.
The reason for this is that we have added a new vanishing point. This point is located far above or below our picture. You will never see this in your photograph. It is too far away from the horizon line. On a very rare occasion at an extreme angle, you might see it in the picture plane, but this is not common.
Anyway, here is an example of a high vanishing point.
Again, the rectangle is your picture. Notice also that the entire horizon line has shifted out of the picture.
For low third vanishing points, the horizon line would be above the picture, and the third point would be way below the picture. (just flip the above picture upside down.)
Here is an example of a building.
The dark lines are the buildings. You can only see the top of the buildings. In real life, this would be like standing on a street looking way up at the top of the buildings.
This is the trickiest form of drawing perspective, and it's not one that you'll have to worry about very often.
I just wanted to make sure you knew what three-point was.