If I complete my B.Tech from an average ranked college, will I be able to do my M.Tech from a good one?

Yes, sure you can do your M.Tech from a much better college than your previous one. For that, you can give some entrance test to get the best college for you. You can give GATE for taking admissions in top IITs, NITs and IIITs. Best of luck.

Yes, that is very much possible. Getting into good university for M.Tech depends upon a lot of factors and not just on the college from where you completed your undergrad degree. Few of such factors include- CGPA, GRE score, TOEFL/IELTS score technical skills, your projects, grade in relevant field