HEIGHT 
Women: minimum 5'6" 
Men: minimum 5'11" 

AGE 
Though age doesn't matter much, I would like to day that few Indian women have started as late as at the age of 26 such as Michelle Tung. But ideally :
A young woman starting out must be from 15 to 22 years old.
A man must be 18 to 25. 

WEIGHT 
There is no perfect weight! But a model must be slim and well proportioned. Depending on the trend or the market, a magazine may opt for a slender silhouette or a more curvaceous look. Regardless, proportion is all-important. A slim-waisted woman with long legs will always be more photogenic than one who is the same height but has a longer torso and heavy legs. 

As for male models, they have to be athletic and muscular, but shouldn't look like body builders. Again, proportion is the key factor. 

GENERAL CRITERIA
It is impossible to specify individual facial features, since a model's character, beauty and above all photogenic qualities are the sum of them all. Healthy skin, well-cared-for teeth and natural, well-cut hair are generally sought after. 

PHOTOGENIC QUALITIES 
This is the most demanding criterion. Many pretty girls do not meet this ultimate requirement - a photogenic face. And yet certain girls whose looks are ever so slightly ordinary or even somewhat unusual can be highly photogenic. This criterion is the most subjective and the hardest to evaluate. Only a modelling professional can give you an accurate appraisal of your photogenic potential. Only 5% of the people  who meet the first four criteria will also pass this final test!  But physical appearance isn't everything. You also need a certain temperament to be a model. You should be aware that it's an exciting but highly demanding line of work. Yes, there are more criteria you'll have to meet. But I'll let you be the judge of them. 

You must have POISE 
A model meets many people in many situations - auditions, photo shoots, fashion shows, go-sees - and in different countries. It's important to be at ease, to be able to communicate easily and to radiate self-confidence. An outgoing person with an appealing smile will always receive more work than someone who is shy and withdrawn. 

You must be able to TRAVEL ALONE
Everyone loves the idea of travelling. But travelling on business is a far cry from going on vacation. A model travels alone, without relatives or friends, for one to three months at a time to distant states / countries with different cultures. This is one of modelling's most  difficult requirements. It takes a great deal of character and independence. This type of work is not for everyone. Unfortunately, you cannot work as a model without travelling. To be selected for assignments, you need a varied portfolio of work done around the world. Successful models love their work and become adept at living out of a suitcase. 
You must be comfortable being by yourself for long periods of time.

It is important to be SELF-RELIANT 
Wherever models are in the world, they must remember that they are working on the road and that a client, a photographic team or a fashion show is waiting for them. Models have to be organized to ensure they arrive punctually in places they may barely know. It isn't hard to imagine the many reasons a model has to be self-reliant. 

Organized
A model must be organized in order to keep track of the different dates, times and locations of appointments so that she can show up at appointments and go-sees on time.

Ambition
A model goes after her goals. You won't become a model by sitting at home and waiting for agents to come to you! (Note: Never sign with the first agency you visit. Always go to at least five before making a decision!)

A model has to be very DISCIPLINED 
Winter or summer, during a downpour or a snowstorm - models can receive an important job at any time. They have to be organized, available and presentable at all times. That means getting lots of sleep, eating properly, and avoiding smoking and excessive drinking. Models must also take care of their skin, hair and bodies with regular visits to the esthetician, the hairdresser and the gym. Finally, they must keep their portfolios and their finances up-to-date and maintain their contacts outside the country. Only the best organized and most disciplined will set themselves apart and perhaps even become top models.

Intelligence
To succeed, a model must be an intelligent business person. She must know how to spend and invest money wisely. Also, she must know how to conduct business with agents, photographers and clients. She must appear professional and businesslike at castings and go-sees.

Confidence
Clients are looking for someone who is self-assured and whom they think can sell their product. A model must be able to sell herself before she can sell a product. It is also important to be able to handle the rejection that every model must face. No model could meet the expectations of every possible client. It is always best to go into a casting not caring whether or not you get it. That way it's a pleasant suprise if you get the job but you aren't let down if you don't get it.

Stamina
A model is sometimes on her feet all day in the freezing cold or the hot sun. She must be healthy and in shape to be able to work all day and still look good. Eating healthy, going for walks/jogs and working out at a gym three times a week is a must.

Will-Power
Unfortunately, the  modeling scene does often involve  partying. A model must have strength of character and will-power in order to stay away from  too much wine and  partying. An agency will drop a model very quickly if it becomes known that she takes drink or parties a lot. 

Once you have read what it takes to be a model, and you think you fit the mold, then the first step is to get a portfolio done.