One of the most important things to be aware of when using a telephoto lens is your background. Which isn't to say that backgrounds aren't important with wide angle lenses but the difference lies in what your background is.
Or more importantly how far away your background is. With the wide angle lens increasing the apparent distance between objects, things that are a long way away from the camera appear small and insignificant. They basically fade away into nothing.
When you use a telephoto lens, however, all the background elements become large, close and major elements of your composition. In other words you almost need to be looking at your backgrounds before you start framing your subject.
And to do this you need to be looking off in the distance behind your subject. The longer the telephoto lens the closer that background is going to appear, and the bigger it is going to be.
Just walking a few steps in one direction or the other won't affect the subject too much but can completely change the background. Take this image here. I wanted to show the contrast between the ancient steps of the Jantar Mantar astronomical observatory in New Delhi with the modern shapes of skyscrapers in the background.
The skyscrapers were a long way away so I knew straight away that there was no point in getting up and close with a wide angle lens because that would make the buildings small and insignificant in the background. Yes the ancient Jantar Mantar steps would be huge but I didn't want just the one element - I needed both to be large.
So it called for a telephoto lens. Then the problem was where I needed to stand to get the background buildings in the right place. A few feet to camera left and the only thing in the background would be other Jantar Mantar buildings. A few feet to the right would give me the same problem. Too low and my only background would be overcast Delhi sky. So I needed to be at about the same height as the little girl and pretty much right where I was to get the buildings in the background.
In this way I chose my background first. I scanned behind me to find a place where I might be able to get up reasonably high and at the right angle to get my background in the scene. After I'd done that the subject pretty much took care of itself.
When I started climbing up the stairs of another Jantar Mantar object (building?) in what seemed like a good position I found that going too high would bring too much of the street in the background into view, too low and I had too much sky. Just like goldilocks I found that the middle spot was just right.
Only problem was there was nobody in my picture. So I just sat down and waited until somebody walked into the frame. Luckily for me this little local girl wandered about half way up the steps, directly opposite me. Now I had my modern background, my ancient foreground for contrast and my bit of human element to make it interesting.
So remember the first golden rule of telephoto lenses - your background objects may be farther away than you think and may appear larger than in real life!