Why is my iPhone sending text messages instead of iMessages?

Go to Settings>Messages and make sure that iMessage is activated. iMessage will only send to other iOS devices. The iPhone will send SMS if you are sending to non-iOS devices. You also need to make sure that you have data turned on, and be either connected to Wi-Fi, or cellular data.

When you checked the area I asked you to look at, does it show that iMessage is activated? What is set for Send & Receive? Do you see a checkmark by your phone number? Does this failure to send as an iMessage only occur for a single contact? If so, then delete the conversation and start over again. I've had iMessages fail to send and then send as a text, but that has never affected the ability to send to the same recipient a short time later as an iMessage.

Were you only having issues sending iMessages to one person? Or to all iPhone users? I am not able to send iMessages when connected to data, only when connected to wifi.

Good to hear.