My iPhone is an iPhone 5. Can I use it to set up my watch?

To set up a new Apple Watch, you need to have iPhone 6s or later with iOS 15 or later. You can not set up your Apple Watch with iPhone 5.

Set up your Apple Watch - Apple Support

irmworm Said:

"My iPhone is an iPhone 5. Can I use it to set up my watch?: Can I use an iPhone 5 to set up my new iPhone watch?"

No.

1. Go Here: Set Up your Apple Watch - Apple Support
2. As it reads: "What you need: To set up and use a new Apple Watch, you need an iPhone 6s or later with iOS 15 or later."