On the exam day, you should only carry your admit card along with you to the exam centre. If you are appearing in the offline mode you can also take a clip board along with you. However you must ensure that nothing is written on it. As per the guidelines for JEE Main given by CBSE, you will not be allowed to carry a watch. All the exam hall will have a wall clock.

There are many instructions that you have to follow on the JEE Main day. I have a provided a link below where you will get all information to be followed on the exam day. So please make sure that you go through each point beforehand to avoid any sort of hassle.