Defender found possible malware, how do I actually get rid of it?

Open Windows Security, Virus & threat protection, then Protection history. It shows what it found and what it already did - most of the time the file is already quarantined and it's just waiting for you to pick Remove from the actions. After that run a Microsoft Defender Offline scan from Scan options. It restarts the pc and scans before windows loads, which catches stuff that hides while you're logged in. If the same thing keeps coming back, look at the file path it reports - a browser extension or something in startup is usually what's reinstalling it.