how do I print my text message?

If you have a macOS computer, see -> How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac -> Forward SMS/MMS text messages from your iPhone to your Mac or iPad – Apple Support (UK)

or you can Use Messages on your Mac.

See also -> Set up iPhone to get SMS messages on Mac

Set up iPhone to get SMS messages on Mac – Apple Support (UK)

It is not possible to a Windows OS computer. 

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If you want to print text message:

If you have a macOS, see the Apple articles below:

Use Universal Clipboard to copy and paste between your Apple devices

Use Universal Clipboard to copy and paste between your Apple devices - Apple Support

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac – Apple Support (AU)

From the Mac, you can just copy and paste the text into Pages then print it out, or Save as . . .

Finder -> Apps -> Messages -> select a messages -> copy -> paste to xxx.

If you have multiple messages, you must select, copy & paste per message.

You can also use a 3rd party app (for macOS and Windows OS), such as iMazing.

It's a paid app.

There are also other 3rd party paid apps available.

jbrakey wrote:

I need to print c text conversation from an ex for legal purposes, how do I print it?

Raicya gave you excellent advice. I think the easiest of the suggestions is a third party application such as iMazing.

However... before you do anything, talk to your attorney. If you mishandle things, you may destroy the evidentiary value of the texts.