I wasted my time in class 7th. Can I still crack the JEE?
You are thinking too much. You are just in class 7th. When I was in class 7th, I did not know about JEE. I came to know about JEE in my class 10th. Most students start their IIT JEE preparation from class 11th. When you are on which standard then first focus on your studies for this class. After that if you get extra time then you can prepare for other things. As every step is important, you can not jump directly to the upper level. You can watch videos on youtube that are related to JEE topics and have in your syllabus for this class. Try to learn them in depth. You can try to solve Olympiad problems. If you want then you can talk to IIT JEE toppers and listen to their experiences when they started their preparation and what was their strategy.