how do i send a text message on an iphone?

ry this --> If you can‘t send or receive messages on your iPhone, iPad, or iPod touch

You haven't mentioned what iPhone model you possess? What iOS version is on that?

Are you facing this difficulty over WiFi or cellular service?

What kind of Plan do you have with your Carrier (Data, SMS, MMS, Voice etc)

Have you enabled SMS and MMS (See the pic below)

SMS or iMessages?

If it's green, that means it's an SMS or MMS message.

Contact your cellular carrier. That's a carrier feature.