Earth probably had a similar atmospheric composition to Venus after it formed with vast amounts of water, carbon dioxide and smaller amounts of nitrogen plus traces of other gases. So the real question is what happened to make Venus different from Earth?
Firstly water can be split by UV light in the upper atmosphere into hydrogen and oxygen. The oxygen would have reacted rapidly on Earth and Venus forming oxides and carbonates on the surface the hydrogen would linger for a while but eventually escape. Over geological time water levels on both planets would have decreased significantly.
But the really big difference between Venus and Earth was the emergence of photosynthesis only on Earth (perhaps Venus was too hot). Photosynthesis saved Earth from the fate of Venus by flooding the atmosphere with a lot of oxygen. The UV splitting of water still happened as before, but in most cases any hydrogen formed that way would react fairly quickly with oxygen before it could escape (it has been calculate that today a mere 3 million tonnes a year is lost so it now takes roughly 4.5 billion years to lose 1% of the oceans.
In time photosynthesis on Earth consumed almost all of the carbon dioxide in Earth's atmosphere converting it to the vast amounts of oxygen we see today and leaving behind vast amounts of carbon in the form of coal, oil, gas and more importantly (>95%) as a 1-2% carbon based impurities in sandstone and other sedimentary rocks (there is an awful lot of sandstone on Earth).
Venus lost it's water due to UV reactions in it's upper atmosphere, the escape of hydrogen and the reaction of the oxygen with the crust. But it never lost it's vast blanket of carbon dioxide that remains to this day swamping the nitrogen.
Earth didn't lose much of its water, it was protected by the formation of far greater amounts of oxygen by photosynthesis. The carbon dioxide on Earth was consumed by photosynthesis leaving oxygen in the atmosphere and deposits of carbon and hydrocarbons in the crust. Earth appears to have much more nitrogen than Venus but Earths atmosphere is at a much lower pressure due to the loss of al that carbon dioxide so the nitrogen is more apparent.