It is not necessary to join a coaching institute to ace the JEE but you will improve your chance of cracking the exams greatly if you do join one. Main reason to do is that you don't have to reinvent the wheel while preparing for the exam. These tutors apart from being subject matter experts in science, they can also give very useful tips and tricks to face the exam, which can be invaluable to many.
Do you research. Ask your friends where they went to for coaching and compare three different centers before choosing one. I will personally recommend individual tutors with proven track records than big brands where you don't know who will be tutoring you. Some big name players in IIT coaching like FIITJEE and Akash have some good centers in few cities while not so great in others. So, knowing the tutor who will directly take classes is important. I will also recommend someone courses by former IITians, who can actually aced the exams and can speak with authority on the subject.
For example, AhaGuru Dr. Balaji Sampath (All India Rank 4 in IIT JEE 1990) is doing both on-class and online tutoring, which is very popular. Here is someone who have scored All India Ranking of 4, which is as tough as it gets, so I would not think twice to put my kid here in his class. We all have our choices, choose your wisely as it is first important step in your quest to get admitted IIT.