focus on ur Btech first of all and maintain a good academic record for 4 yrs!!! and then either from third yr or fourth year as u wish start preparing for GRE!!!!!! u must take care to consider some important points --
1)which university in US is best for the MS u want to pursue(every university is not good for evreything)
2)what is that particular universitys reqiurement for that course like whther it requires TOEFEL or not!!and letters of recom etc!!!
and such things should be enqiured thoroughly !
and now give GRE and then try to score i.e give ur best!!!!!and then proceed accoedingly!!
but first focus on Btech civil and maintain a good academic record and ALSO have a good repo amongst the prof and etc!!! and then give GRE in 3 or 4 th yr as u like and according to ur prep!!!
and as u want to pursue MS in Physics ,make sure u do a lot of research in Physics during ur 4 yrs of Btech!!!!
ALL THE BEST!!!!!