The most widely accepted theory is that the planet Venus was formed through a process called core accretion. In core accretion, gravitational forces draw together dust and particles from space to form a rocky core, which in turn gravitationally captures the lighter elements which compose its atmosphere.
Scientists estimate that Venus formed over 4 billion years ago. Although Venus is similar to Earth in size and has a similar core, its closer proximity to the sun caused the planet's atmosphere to form differently from that of Earth.
Studies indicate that most, if not all, of the water on Venus evaporated billions of years ago. The resulting thick cloud layer which surrounds Venus is comprised primarily of carbon dioxide and is 100 times more dense than the atmosphere of the Earth. The greenhouse effect caused by this cloud layer has also significantly increased the temperature of the planet to the point that today the surface of Venus is hot enough to melt lead. All of this makes the possibility of landing humans on the surface of Venus highly unlikely as the air is not breathable and the weight of the atmosphere would be enough to crush a person's body, assuming she could withstand the heat.