Use iPhone to text Android phone

First, check to make sure your cellular plan includes texting. If not, as SrvanKra advises, establish one. Next, go to Settings->Messages and make sure Send as SMS is turned on as well as MMS Messaging. They should be on by default. Messages sent from an iPhone to non-iphones users will appear with a green background color. Messages sent to another iPhone user will appear with a blue background color.

Please get an SMS plan enabled by your carrier for the SIM you use.