Mechanic Q: how to fix emission problem??

everything resets when ever the key is turned off, same as if battery is removed, so computer is never able to complete checks so it alwys repoirts most systems as not ready, told I needed to replace $1000, computer, saw several for a lot less but not sure if it would correct proble, or if simpler cure, or even how to replace the computer, never hwad trouble with a computer before.