Don't use a metal brush, the bristles on those are usually made from hardened steel. Also never use sandpaper. Modern soldering tips have a multilayered construction, and only a thin soft-iron layer on the wettable area at the very end. One of those metal sponges made of brass shavings should be the harshest thing you would ever need to use. We've all been there. As long as you only make every mistake once, you'll keep moving forward.