First of all, to show your art at a farmer’s market, street fair or gallery you need a few things for legal reasons. The most important is a seller’s permit. Some places will also require you to carry liability insurance. How to go about getting these things will be discussed on another page (for now you can find some info here: Legal/Business).
Getting into street fairs is usually quite easy. Usually they have a website and you would fill out an application. It is usually relatively expensive and you need to apply months in advance to ensure you get a spot. The popularity of it will usually determine the cost. For example, there is an Art Walk in San Diego that thousands and thousands of people go to every year. It costs over $400 for a space. Another local street fair I know of only costs about $200 for a space.
Farmer’s markets can be a little more difficult to get into sometimes. Sometimes there is a long waiting list if they have no space (a local farmer’s market in San Diego has a waiting list of over a year!). To get into a farmer’s market I would recommend going to their website and filling out a vendor application, then taking it in person to the organizers of the farmer’s market.
Farmer’s markets usually run once a week and cost anywhere from around $25-$60 per week to have a space. You’ll need to bring your own tent and/or table and some important recommendations for past experience are bring chairs, water and snacks for yourself and anyone you have helping you out.
For tips on hanging your art, check out this page: http://www.theartcollaboration.com/putting-on-art-shows/hanging-your-art