I am currently pursuing my B.E. in E&TC,presently in third year.how can i join the army after B.E.?at what post? arms designing? procedure?

It requires some eligibility criteria like age, and all So first you need to pass this UG then you can try for it.

Apply for CDS exam to join Army as commissioned officer. For complete details visit www.joinindianarmy.nic.in. By the way, you are sadly mistaken that you will be designing arms at Army.