Hi Aspirant,
After your B.Tech,you have the following options If you want to get into the army-

TGC (Technical Graduate Course): The most popular entry for engineering students to join Indian army after engineering is TGC entry, TGC entry is conducted by Indian army two times every year and engineering candidates get shortlisted based on their percentage till 6th semester. TGC entry is only for male engineering students and one gets permanent commission through this entry, one who clear TGC entry interview join Indian military academy for training.

Also,If you're younger than 25 ( 24 for army),then you can go for CDSE ( Combined Defence Services Exam). This exam gives you opportunity to join all 3 forces ; Army, Navy and the Air Force. You'll have to give the SSB Interview if you clear the MCQs type exam conducted by UPSC twice a year.

Hope this helps.