You don’t need to worry as you have around one and a half year to prepare and trust me it is enough for anyone to prepare for the JEE. If you work hard and don’t waste your time in the future you’ll achieve whatever you want.
If you are dedicated enough, stop thinking and start working. This is the high time you have to focus on the preparation.
Start clearing your basics for the each subject of JEE Main 2018. This will help you build the foundation and it will be advantageous in Class XII.
Another thing you need to keep in mind is study according to the timetable and proper planning.
All the best!!!!