Looking closely through the viewfinder may reveal things that you might not have noticed, even if you don't get the picture. Reading books will enable you to start out well informed. One of chief considerations is the size of the thing to be photographed. Objects can range from hills and mountains down to microscopic objects. Since they all have to fit on to the film, the magnification must vary inversely with the size of the object.
Magnification affects a very important variable - the depth of field.
In practice, there is some latitude, because imperfections in focus can be tolerated up to the resolution of the film and lens, if not more, depending on the subject and the requirements of the photographer. One problem with magnification is that the greater it is, the narrower the depth of field. Many objects are about as deep as they are wide. A mountain which is a kilometre wide may have a base which is a kilometre closer than the peak, but this never troubles the photographer. But a fat spider, photographed at high magnification, can be very difficult.