You must design a valid experiment to show that there are microorganisms in the air. what would be your dependent variable?
sterilized broth
growth of organisms in the broth
exposure to microorganisms in the air

growth of organisms in the broth i hope this helps

You can carry out a test , 
1- take a sample of the air in a test tube 
2-add limewater to the test tube 
3- result : if it turns milky/cloudy it has microorganisms 
4- reason: because every microorganism respires , and so it produces Carbon dioxide . 
and limewater is the test to show the presence of Carbon dioxide .