The job that you're applying for is presumably a part-time job or a short term contract. The employer already knows this so you don't have to tell them that that's what you want. If you are writing a cover letter to go with your CV, you could include why you only want a short term or part-time job in that, presumably because you are still a student and therefore are only allowed to work a certain number of hours a week, etc.
In terms of interests, you must have done something. Interests include anything beyond schoolwork and employed work/volunteering that you chose to do. It could be playing an instrument, being part of a choir, dancing, running a youth group, being politically active (eg campaigns, leaflet drops, etc), doing a First Aid course, participating in a school play, fundraising, etc. Basically, it is about anything that shows that you don't just go to school, come home, sit on your bum and watch telly all evening.
Each interest that you have has to show a quality of your personality as well. For example, in my partner's CV, she mentions how she passed Grade 8 ballet with Distinction when she was 11 and Advanced 2 with a high Distinction when she was 15 (2 of the highest awards in the ballet world at the minimum age allowed). This showed how she was determined and dedicated and saw an objective through to the end, achieving very high results.