How did the atmosphere formed ?

The atmosphere is the skin of air around the earth. The quick version is that the first atmosphere came from volcanoes and was mostly water and carbon dioxide.  As it cooled down it rained and made the oceans and a lot of the carbon dioxide dissolved.  Later some types of algae started making oxygen, until eventually the atmosphere was like it is today.
When the earth formed it was so hot the rock was molten, like lava all over.  There were lots of chemical reactions that made gases, but they were stripped away by the strong wind that comes out of the sun (solar wind) very quickly.  When earth cooled a bit it formed a crust but there were still lots of volcanoes, gases came from them to form the first atmosphere.  It was mostly water – the earth was so hot there were no oceans and the water was all steam and clouds.