How do I turn on iMessages?
I have an iPhone 5.
iPhone SE, iOS 18
Which do you have? An iPhone SE (2016) or an iPhone 5 (2012)?
The iPhone 5 must have iOS 10 installed before it can use iCloud and Messages:
System requirements for iCloud
Then follow the instructions in Set up Messages on iPhone
If you have problems setting up Messages see If you can’t turn on or sign in to iMessage or FaceTime on your iPhone