Question

How is pressure developed in a container full of gas

Answer 1

Pressure, is developed by the gas molecules hitting the sides of the container walls. When the gas molecules rebound (assume ideal gas), it exerts a force on the container walls due to a change in momentum. The force acting on a unit area can be said to be the summation of all the individual forces of every molecule that exerts on the container surface. 

Using the ideal gas equation which rearranges to P=nRT/V, 
to develop a huge pressure in the container you can try to put in more gas, so that more molecules can exert a greater force on the wall for that same area (increase n), increasing the temperature (T) or reduce the volume (V). 

Answer 2

Gaseous molecules are free to move in the container.The molecules collide with each other and with the wall of the container.On the wall the molecules exert force. The force per unit area is called the pressure of the gas.