I would like to answer your question in 2 parts. In the first part, I will say you how to get a career in cricket. In the second part, I will give you few suggestions (not advice). After examining my perspective deliberately, choose what your heart says.
Part I - How to get a career in cricket?
I mean that the career you need is a place in the Indian national cricket team. With that perspective, I would give you the following suggestions.
1) You have to decide firmly what role you are going to play in a cricket team. I mean whether you want to be a batsman or bowler or an all-rounder. If you want to be a batsman, then you have to choose your order. I mean whether you want to be an opening batsman or middle order etc.
2) Before deciding your role in a team, deliberately examine it. For example, if you want to be a wicket-keeper batsman, then your chances are very low as you will be having a very heavy competition. So, tailor your choices according to the needs.
3) After deciding your role, you need to master it. For that, go and join the best cricket training institute in your area for proper guidance. Because technical skills are well taught by cricket coaches.
4) Practice hard and show your skills in the local matches so that you will be selected for the district level matches. To get into the district team, once a year, a selection camp will be conducted by the district cricket association. Show your skills in that camp and get selected.
5) If you get into your district team, chances are high that you will be selected to the state team. For this, you have to outsmart all other players of your department.
6) If you get into your state team, your next aim should be the national team. Use the Ranji matches, trophy matches to get into the limelight. If you become the latest sensation of the media, automatically you will get the call from the national selection committee.
To achieve all these, you should be built on 4 pillars. Those 4 pillars are hard work, sacrifice, commitment, and patience.
Second Part - Eye openers (questions to you)
1) You have said that you are already 21 years old. Now, if you dedicate your life to cricket, you need to practice at least 6 to 8 hours a day on the ground. So, your entire life should be clocked with cricket. Do you have enough time? Can you toil hard on the ground?
2) Currently, the Indian team is well equipped with quality players. Players like Virat Kohli, Ajinkya Rahane, Rohit Sharma, Lokesh Rahul, Ravichandran Ashwin, Ravindra Jadeja etc are in their 20's. It clearly means that they are going to dominate the field for around another 5 to 10 years. So, how you are going to outsmart these players to cement your place in the national cricket team?
3) You need to spend a lot on cricket kits, nutritional foods, training and physio. Are you financially stable?
4) Will you be supported by your family to achieve your dream?
5) You are already 21. If you have to make a career in cricket, now, you should be playing in your state Ranji team. If we analyse practically you would need around 5 to 6 years to get into the national team. So, you would be around 27 by then. Will you be financially stable until then?
I have asked these questions not to demotivate you but to push you into a deliberate thinking. Before taking any decisions you should analyse its advantages and disadvantages. You have to analyse your strength, weakness, opportunities, and threats. Only then, you can make the right decision. There are differences between desire and dream. Yours could be a childhood infatuation on cricket. So, choose the right path after deliberate thinking. I hope you will take the right decision. Good luck. But, one request from me to you. If you don't succeed in your dream, then please don't push it on your children ( I mean it in the future).