First, to answer your question. Human activity is solely responsible for global warming being a problem we need to solve. As @HungryGuy & @tedd noted, the Earth routinely goes long glacial periods interspersed by shorter interglacials where the temperatures moderateglaciers recede. We are in an interglacial now, and it started about 10,000 years ago.
Atmospheric CO2 levels track with these temperature fluctuations. But in the last 500 million year, atmospheric CO2 has peaked at about 300 PPM. It is now approaching 400 PPM and ramping up.
To your other question, how much is human activity and how much is natural. This is one of the tools the #37 trillion a year fossil fuel industry uses to foster junk science in order to protect their profits. The truth is that most of the CO2 that goes into the atmosphere comes from natural sources. Animal respiration, decay of dead animal and plant matter, naturally occurring forest fires and volcanoes all put CO2 into the air, much more than humans add. However, the earth’s natural CO2 absorption systems, oceans, rain and plants, remove this CO2 so it doesn’t build up unduly.
Final question. What arte humans doing to cause CO2 buildup. We are brining fossil fuels and plant material, adding an additional 29 gigatons per year and that rate is constantly ramping up. And we are rapidly deforesting the Earth, removing one of the natural sinks that pull CO2 out of the atmosphere. CO2 in the atmosphere has a natural half like of roughly 7 to 10 years. But when we are pumping in additional CO2 the the Earth’s natural sinks can’t absorb, what we add has no half life. It stays and accumulates year by year.