Being an Air hostess is a very tough job. It is very demanding job. You need to be completely focused in order to become a successful air hostess.
Normally, there are separate rules for becoming an air hostess in different parts of the world. However, some general rules are given here. In order to become an air hostess most of the countries require either a High School Certificate which is A Levels or Graduation. Some major airlines also prefer candidates who have a diploma in tourism or hospitality management.
Then you can apply to a specific airline, who conducts their own training or join an air hostess training institute. It is always better to opt for an airline because then recruitment is no problem for you otherwise you would have to apply for a job.
Remember that you must have a positive and friendly attitude. You must be patient and willing in your heart to serve others.