Finding a hobby is very important, I think. Otherwise, it is possible your lack of interest would spread, and you may stop doing things you are responsible for.
That is what happened to me in my Junior year of high school. I stopped doing homework and received horrible marks.
I would recommend learning an instrument. I started with the guitar and since then have moved on to the piano and violin. So something like that might be good. Music, reading, writing, drawing, video game programming... you never know what you will like until you give it an honest attempt.
You can start by joining sports clubs and gradually you will create interest. You can also decide to go to the beach if it is possible or play computer games at home. This will enable you to become a cheerful person, which is also a positive influence to your academic performances.