If you want to do MS in a top american tech school then you should start working on your profile right for mid 2nd year or 3 year.
The way tech schools select students is quite confusing some times rather not transparent becoz suppose u and you friend apply and you have a higher GRE and CGPA than your friend but your friend would have been selected tats because he might have had research experience in the field the tech school is looking upon

generally your profile is weighed on the following :
40 % G.R.E
20% paper presented and work experience
40% c.g.p.a and research experience

and some university s give very high weightage for reccommendation letters so keep good rappo with your dept hod and other senir proffs

so if you dont have a great cgpa its better you start working on the topic of your interest ( I would suggest you to go to the Research and development section of americas top univs webpage so that u will get an idea of research going on there

and other thing is you can also find the proff who is currently working in your are of research from their website and mail them about what you have done this will significantly increase your chance of getting admitted