It depends how old you are.... if you want to work in a games store or 7-11 you'd at least have to be 17/18 I'd assume...
Ive just helped by 15 yo stepdaughter come up with a resume for a job at a local dept store. She interviewed last week, and got the job, having never worked at a dept store previously.
What you need to do :
get together a resume, including a little cover note about who you are and why you are interested in the job.
If you dont have a long work history, include on your resume
*achievements - any school, sporting etc achievements
*areas of knowledge and interest...
in an interview, be polite, be yourself. Dress neatly, ie no shirt tucked out etc, look presentable
answer questions asked as best as possible. Don be afraid to admit things you may not entirely be familiar with. If asked questions about 'how would you handle this scenario'.... try and relate it to something similar that may have happenned to you