How to activate an iPhone 5C for Boost Mobile?

It's a Sprint iPhone 5c, new, but when I go BoostMobile's website and enter my MEID into the "identify your phone" page it tells me, "MEID/ESN provide is not eligible for activation with Boost. Please use valid Boost equipment for Activation."

So how do I activate my phone then? Any help is greatly appreciated.

You need to call customer service. Sprint needs to release the phone so that Boost Mobile can activate it. Call Sprint first, then Boost Mobile.