how do i attach the airtag to my iphone?

There are a number of options you can choose from ... either through the Apple Store or other resellers. However, you would not necessarily want to attach it directly to your phone as that would defeat its purpose. As you would need to use your iPhone to track its location ... or depend on other people's phones to do so.

Your iPhone is defined as a "device" by Apple; the AirTag, on the other hand, is an "item." Both support Find My, so again, having both together would not be very helpful. IMHO.

Ref: airtag accessories - Apple