Question about studying for summer exams, is this a good strategy?

I used to be a really good student with only A and B grades, until school got harder as the academic years went by and I've noticed my grades dropped also because I stopped putting in effort like I used to. I have final exams which take me into college starting from May and going into mid June. Because, I want to do very well and be more natural when it's time to do exams, I've started studying already which means I'm studying six months prior to the exams. Will this be enough time to be able to do well? I'm so scared about not doing well because these grades will be a stepping stone into the next transition which is college. How can I do well, and how do I make studying more fun?