Check the FAQs and guides (linked from "all about rooting" sticky post, which is the red link in my sig).
But basically you need a ROM Update Utility (RUU), which you download from
Shipped ROMs. Note, that link is for GSM Desires - CDMA are phone model BravoC, which is a different link. This is a Windows program which you run with your phone connected via USB (in fastboot mode if you have changed your ROM). It will wipe your phone and load stock software.
You will need to find the RUU corresponding to your network. If you have trouble identifying it, let us know what you are looking for.
P.S. I know some of us (myself included) are getting on a bit, but normal sized text is still OK for most of us.