how can I apply for Tata motors .........btech in mechanical engineer

I will interpret the word “how” in 2 ways -
What is the procedure to get into Tata Motors?
What skills do you need to get into Tata Motors?
I will answer both questions considering that you are looking for an entry-level job.
Procedure -
Tata Motors hires fresher Engineers for the post of Graduate Engineer Trainee through campus placements. It visits campuses of new IITs, all old NITs and other reputed colleges from different states, like College of Engineering Pune in Maharashtra for recruitment. GETs had an initial probation period of 1 year after which they were evaluated and given a permanent status with the title Assistant Manager.
Until recent times, Tata Motors also hired fresher Engineers from colleges it does not visit for campus recruitment, if they were referred. The hired Engineer got the title Support Service Engineer. Such people were given a 1 year contract which could either be extended or ended after completion. If their performance was good, they would be given permanent status and the title Assistant Manager (similar to GETs). A contract could be extended for 3 more annual terms after the completion of the first year without being awarded a permanent employee status. The contract could not be extended beyond this period.
The situation now is terse. The company has undergone an Organisational Effectiveness exercise owing to which the hierarchy has shrunk from 13 levels (Assistant Manager to Executive Director) to 5 managerial levels (who get designations like Head XYZ where XYZ is the area/department they will control) and 3 independent levels (with no designation). The main aim obviously is to offload extra manpower and save money, which has been publicly admitted by HR.
What it means is that the Support Service Engineer hiring will totally halt, or if it doesn’t, the chances of becoming a permanent employee will be zero. So the only chance to join Tata Motors will be through campus placements.
Skills -
Of all my classmates who applied to Tata Motors during campus placements, I was probably the weakest - average academics, average communication skills, poor technical knowledge, no participation in co-curricular activities related to Automobile, like the popular student competitions BAJA and SUPRA. I still maintain the opinion that I got lucky(?). However, my 2 cents from my interview are that you don’t need to be an Engineering wizard. The questions were pretty basic and needed nothing more than logic. Just stay calm, listen carefully and answer confidently. I’d say be aware of regulatory changes in automobiles (like the implementation of Bharat Stage IV emission standards in all commercial and passenger vehicles). I don’t have any success mantra to tell you in this regard.
Do get back to me if you need to know anything more specific. I will be happy to share whatever I know.