This problem has been reported many, many times. Basically it usually has to do with the imessaging service. It can't communicate with "regular" text message recipients. Please search the forum (use the "filter" box on the main forum page) for "iphone" or "imessaging," and you'll find other threads where it's been answered. Or look at the "More Like This" column to the right of this thread.
It appears you have moved from an iPhone to an android phone. The problem you are having is because you did not deactivate your iMessage service before leaving.
If you still have your iPhone, you can re-activate it and then
If you don't have your phone or don't want to go thru the hassle of reactivating it, you can simply call Apple at 800-694-7466 and ask them to turn off your iMessage.