How can i start working a side job

Make sure you have a resume!! if you don't have a resume, then you probably won't get a job lol

If you're unsure of what one looks like then you can easily look up some examples! I'm assuming you haven't had a job before, so instead of listing job experience you can list some places that you've volunteered at if you have! 

Once you get hired, or if the manager/interviewer asks when you're available make sure you can take shifts after school or on weekends

It's 100% ok to tell potential employers that this will be your first job if hired. But yes, you need a resume and a cover letter. List any and all experience you have outside of work (school activities, clubs, volunteering, unpaid labor, etc)