Wow, my computer is so slow, all these other responses showed up while the page was loading!
Cathi, there's some temporary stuff you can get in the beauty supply store that'll make it darker until the next time you wash it. It comes in a gray bottle with pink writing but I can't remember the name of it. For a dramatic change, from blonde to brown, or if you're unsure of the best color or how to do it, I'd say go to a salon the first time - I know it's expensive, but you can get good professional advice for the first time, so you can do it yourself the next.
My hair is naturally a light auburn color, but it fades out to blonde in the sun. When I started staying out of the sun my roots started growing out, and it looked like I had been bleaching my hair. I went to a salon for the first visit, to help color all the hair to match the roots. Too funny - I started coloring my hair to keep from looking like I colored my hair!
But now I like to make it a deeper red, it's fun though it does wash out way too quick, but permanent color is way too damaging - I'll stick with the semi-permanent for fun every now & then. I use the Natural Instincts Desert Sunrise, but it's getting harder to find. Hope it's not being discontinued!