howto get a better college in b.tech andhow much rank we will get

If your rank is in between top 1000 students in JEE advance and in between even 10000 students in JEE mains you can expect good colleges for Btech.

It depends on your choice whether you want  a IIT for B.tech or a NIT college

To get into the top 7 iits your jee advance rank should be below 600 or 800 for computer science and below 5000 for civil engg , below 2000 for electrical and below 3000 for mechanical engg. To get into the top nits your jee main rank should be 10000 to 15000.