Hello there. For joining any course in IIT, you need to qualify the JEE Advance exam. After you have qualified the exam, then you have to register for JOSSA which is a platform for counseling of all the government colleges across India. According to your rank in JEE Advance, you can fill out for the choices for college in which you can select the IIT having automobile engineering and then if your rank is enough, you will get the seat otherwise you have no other option to get into IIT.

And for this year all those activities are already done, so good luck for next time.