Did have an iPhone before? If yes, did you use iMessage? If yes, did you sign out of iMessage when you switched to android?
If you didn't, it will take 45 days till you will be auto-logged out of iMessage. If you wish, you can contact Apple to let them do this manually.
If you're problem is not iMessage related, ask your friends to long hold the blue textballoon till they can choose for sending the message as regular text message..
That should solve your problem.