You can fill your application form for JAM Exam till 10th Oct 2018.
You must check the complete procedure to fill the IIT JAM Application Form first. Otherwise you may face problems while filling your form.
So, first check the procedure from the above mentioned link and then apply for JAM Exam.
IIT JAM Exam will be conducted on 10th Feb 2019. So, prepare yourself well.
Best of Luck!