I erased the hard drive on my Mac desktop as someone attempted to install malware on it. How can i reinstall everything without putting the malware back on it? I saved all my info to an external drive before the erasure.

It would have been easier to get rid of the malware, but too late for that. For what it's worth, Malwarebytes is a free app that is safe and works well. The Mac OS can not be infected with any virus, so don't panic next time. If you do not have an OS on your back up drive, you could reinstall your operating system: restart in Recovery Mode by holding down the Command + R keys at startup. This will give you a clean OS. Once that is completed and set up, you can use Migration Assistant to transfer your files. Are your files backed up withTime Machine? If so, then connect your Mac to the backup disk with a USB cable and use Migration Assistant to transfer the Time Machine files. If you did not use Time Machine, but another backup app, it may have a restore function. Otherwise, you will have to drag all of your folders/files over. Good luck.