I can see you are already on a good track with this because the theory is that humans and monkeys do have a common ancestor (one did not evolve from the other).

Apes (such as chimpanzees, orangutans, and gorillas) are more like us than monkeys are, but we did not evolve from them either. We just have a common ancestor.

The reasons we think this is true include both the genetic similarity between humans and other primates (monkeys, apes, etc.) and also because of the fossils that have been found. Rocks are laid down in layers, so we generally find older fossils deeper than newer ones. Human fossils have been found in layers down to those about 200,000 years old. In rocks that are lower than that, there are no human remains, but there are fossils that are like humans in some ways, but have smaller brain spaces, heavier brows, and other differences.

Even deeper fossils show adults that were smaller than we are and had smaller brains than a person of the same size would have. These individuals were not apes, but they were not actually human either. They belong to the same family we do, however: the family homonidae.

By looking at the bones of these member of the family homonidae, scientists can tell that they walked upright rather than walking on their knuckles like apes. The best known of these early homonids is probably "Lucy," a member of the species Australopithecus afarensis. Her fossils seem to be over 3 million years old. You can see pictures of some of these skulls at the Smithsonian Institution web site at:
humanorigins
or The Taung Baby .

We have not yet found and named a specimen of the common ancestor of both apes and humans. At least if we have found it, we don't recognize it yet. We share 98-99% of our genes with chimpanzees. We share a bit less with gorillas. As you compare humans with other animals, you'll see that we share more with primates than with dogs and more with dogs than with fish, and more with fish than with snails, and so on. We interpret these differences to mean that we are more closely related to primates than with dogs, more closely related to dogs than to fish, etc.

Darwin's theory of evolution by natural selection states that all current species evolved from previous species through the process of natural selection. Most people think of natural selection as being about the "survival of the fittest." This is a reasonable summary, but remember that "fitness" in biology means leaving the most offspring, not being the strongest or toughest.

If you are interested in studying human origins for a living, you might want to become an archeologist or physical anthropologist.