A. You are in your final year of B Tech, Civil Engineering. You also mention that you are a diploma student? does it mean that your B.Tech is a lateral entry? If yes, then please check that your diploma is from a UGC recognized college/university.
B. Please complete your B.Tech, study hard to improve your mathematics and other weak areas and prepare for the GATE exams. I say this because once you have good scores in B.Tech and GATE, you stand a good chance in getting admitted into a reputed college for M.Tech which would be a good start towards an academic career.
C. After your M.Tech, apply for the NET/SET exams which is the qualifying exams to join any government teaching college as a lecturer/assistant professor.
D. Once you join, please focus on presenting and publish papers and while you gather experience as a teaching faculty, you can enroll for a PhD via distance education INGOU (http://www.ignou.ac.in/ignou/aboutignou/school/soet/programmes/detail/105/2).
From a lecturer in an engineering college, the promotions are often time bound to assistant professor, associate professor and then professor. For the last two, a PhD becomes mandatory in many colleges.
E. In case age and finance is an issue, you can work in any of the private colleges after your B.Tech and then do your M.Tech by distance mode and then follow the above.
F. Lastly, you need to keep track of the developments in Engineering education, there are many engineering colleges closing down, many streams stopped, UGC and AICTE is being planned to be replaced. So, keep track of this, by the time you complete your M.TEch, see how the scene is, if good pursue your academic dream, if not then quickly get into the private/public sector job.