What one can do to be an airhostess ?? Which college ? What exam are there for it ?

There are no such particular colleges. There are institutes which provide diploma, degree,  and certificate training prohrammes for becoming air hostess. One of the leading institute is frankfinn institute. The link below will provide you all the information abou the institute.

https://www.frankfinn.com/lp/air-hostess-training.php?utm_source=Adwords&utm_medium=Search&utm_campaign=Diploma-Search-Frankfinnn&utm_term=FIAT&gclid=EAIaIQobChMIw5agzaWz4QIVRBOPCh0O7QFeEAAYAiAAEgLCT_D_BwE

More institution you can go for are:-

Air Hostess Academy, Pune
Avalon Academy, Dehradun
Air Hostess Academy, Delhi
Frankfinn Institute of Air Hostess Training, Delhi
Frankfinn Institute of Air Hostess Training, Mumbai
Universal Airhostess Academy, Chennai

Hope you find this useful.