Oksana, thanks for your advice they are always very helpful. I will look up Henri Cartier-Bresson, also to refreash my memory on your video on composition

It is normal that you can feel a bit confused switching from 35mm to 300mm, but practice makes perfect. The more you shoot, the better you will know what to do. 

The way you can think while shooting: First find you subject. Then look at the background. Are there elements which can compliment it or distract from it. Did you see my video on composition: Composition in Photography I am talking in this video about elements which can make you photo stronger - like likes, curves, patters, repetition, natural frames, rule of third... Depending on that, you can choose where to position your subject in the frame and what to include.

Also I doesn't mean that you subject will be in the best position at given moment (like animals in the zoo - you can't control them). What you can do is wait till they move into desired position. Wait for Decisive Moment, invented by famous French photographer Henri Cartier-Bresson. Look him up! Some photographers would sit and wait for hours for that perfect picture. But you will need to kind of know what you are waiting for and what do you expect to happen,where and what time of the day (different type of light might also mike a huge difference). 

So there is two ways in journalistic type of photography: You are lucky and everything will line up perfectly right away and you don't miss it or you might need to wait. There is also a chance it will never happen. But it doesn't mean you should not take picture. At the beginning you might not even know what to look for. Just learn basic rules of composition, shoot a lot and analyze your photos after. It all will come with experience.