how do you put a gas under pressure? how does it react?

Answer 1

1- If the the gas is kept in a container (i.e. a constant volume), you have to provide heat energy,. Temperature will increase , so that pressure will increase proportionally with its absolute temperature.
2-If you you don't have heat energy, you may put the gas under a piston to reduce its volume. The pressure will increase, such that pressure multiplied by volume remains constant.
3- Apply heat and decrease volume in the same process.

Answer 2

Pass it into a compressor. It heats up.

Answer 3

you decreease the volume. its temperature goes up.