Spend some time studying this multi-page website about the application of the scientific method to understand the evolution of life on Earth. An understanding of evolution and classification must come before an understanding of phylogeny.
Organisms are a product of their genetic history.
There are mutations, from time to time, that cause slight changes in the gene pool of a group of organisms. As long as the organisms freely breed within the gene pool, there will be slight variations, but nothing more. If something happens that separates a group from the original gene pool, some of the original variations are lost and new ones arise. Over a period of time, this can cause the separated group to have characteristics that are not found in the original group. A species is a group of organisms that can mate and produce fertile offspring.