A lot of stuff that you might hear in school, restaurants, concerts, museums, parties, bathrooms, and wherever else is a good source of inspiration. Write about it. You'll find a lot of stuff that you feel strongly about and can write volumes on. If you look around you'll find plenty to write about. Investigate places that publish information on zines or that carry zines.
Free press is always good.
Find out about event where people might be interested in your type of zine. Then, if you want to, ask them to submit something. Especially ask people that you see writing and drawing frequently if they would like to contribute to your zine. Put an address on your zine so that people can write to you. That's pretty much it.