I'd recommend working at your barn (that is, if you board/ride elsewhere). That way you can do everything all in one; you won't have to be driven to separate places and will be able to be around horses.
Other than that, my suggestion is freelance. I started my own freelance company as soon as I turned 18 and do freelance graphic design work--I sell websites, am an event photographer, and a writer. The hard part is is that you can't develop these skills overnight and by using google--I've been doing graphic design since 2004 and then graduated and started going to university this year for it.
However if you already have some of those skills it's something to consider :)
Websites like Hubpages pay you according to the number of pageviews/adsense clicks your articles get, if you're into writing.
It's a dangerous business, Frodo, going out of your door," he used to say. "You step into the Road, and if you don't keep your feet, there is no telling where you might be swept off to."