Many people like to have everything in focus in a photograph; this is down to personal preference and subject matter. Zooming in or out lets you chose how much surroundings to the left and right you include with your subject, depth of field lets you chose how much surroundings you keep (in focus) in front and behind the subject.
First off, there is no right or wrong, it is you photo and it is up to you how much you have in focus or not. If you are photographing a beautiful landscape you might want everything in focus from your feet too the horizon but if you are photographing a person in a busy street you might want just that person and not all the other people so that the viewer of the photo knows who the subject is.
So how do you control the depth of field? Well, by using the f-numbers or f-stops, these numbers let you know what the aperture is set at. The aperture is a hole in the lens that opens and closes a bit like the iris in your eye, the bigger the f-number the smaller the hole which gives you more DOF, the smaller the f-number the bigger the hole which gives you less DOF. So big f-number = big DOF and small f-number = small DOF.
As the aperture opens and closes it will let more or less light through the lens, so with a large f-number you will get less light and so you will need a longer shutter speed, and the reverse is true of a small f-number.
It can be hard to see the effects of DOF on the display on the back of your camera, so until you know what results your camera will give you at what setting I would advise you to take a few photos at different f-number and you might come up with a great photo.
Photography is all about trying new ways of doing things and above all have fun.
