This is a big question, and there are entire books on what we have found about the answer. The simplest explanation is evolution/natural selection. This is the idea that the more "fit" an animal is, the more likely they are to reproduce. This concept, over millions of years, causes the changes that ended up with us being humans.

A simple example: suppose there is a species of white moths that live on trees. Birds eat these moths. Randomly, one moth is born brown. This moth will be less visible to birds since it blends in. It has a mutation that makes it more likely to get older and not get eaten. It's also more likely to reproduce. Thus, it will make more and more brown moths until the white ones are all picked out by birds. This is a simple example, but over many millions and billions of years, these changes led to humans. We are very well-adapted! It would take a long time to explain each individual pressure that led to us becoming human over billions of years, but this video watch it from the 1980s TV series Cosmos does an incredible job of explaining it with cartoons. It's really incredible.

There are a lot of facts about how humans came to be that we don't know yet. From what we do know, humans evolved from now-extinct apes living in Africa several million years ago. The exact age is difficult to define, because there are a number of other human-like animals that are also extinct, and we can't say where humans begin.

Our species itself appears to be about 100,000 years old, based on the fossil bones looking like the bones of humans living today. However, many questions are still left. For example, humans display a level of intelligence that other animals do not seem to have, and we don't know how that came to be yet. One idea was that it was made possible by our walking upright and having two hands to use tools with, but this idea has been proven false because we stood upright long before we learned how to make advanced tools.