Hello Student!

For getting admission into a good University for MS, following are certain things that are important:

• GRE score, TOEFL score
  
• GPA/CGPA in all 4 or first 3 years of engineering
  
• Research publications
  
• Project work
  
• SOP, LOR, etc
  
• 
  

First of all, no college will ever ask for your score in entrance exams that you gave in 12th.
If at all, you'll have to enter your board marksheet.

So what you can do from the very beginning of BTech is:
1) Concentrate on college. It's no more like 11-12th. Attend well, score well in all internal and semester examinations.
2) Start preparing for GRE and TOEFL probably in your 3rd year. 3-4 months are enough for preparation.
3) Work on projects. Try and ask professors if you can assist them in any research they are doing.
4) Once you're done, while applying for colleges you need to submit letter of recommendations from your professors. So you'll have to maintain a good rapport with them and also must have worked with them at some point so that they can write more about you.
5) Write a good Statement of purpose yourself, no plagiarism.

Hope this helps :)
Best regards.