To become a Guardian, the only real known prerequisite is that you need to fight in the name of the Last City; that's why many pre-city Lightbearers such as Drifter are not referred to as Guardians (either by choice or circumstance). There's a fuzzy case with Hawthorne though. She was called a Guardian by Zavala in the Red War campaign, and her group was allowed to participate in last year's Guardian Games by representing Titans, so being a Lightbearer doesn't seem mandatory at least as of recently. For the sake of simplicity though, Guardians are often classified as Lightbearers who fight for the Last City of humanity. To be a Lightbearer in general heavily implies that the user must be deceased first, with maybe the cause of death having an influence. The only known exception of a non-revived Guardian is Shin Malphur, but the validity of that could be challenged since there has never been another known case.