Lightly oxidized copper -reddish brown colored- doesn't need much, light sandpaper or scuffy pad would do just fine. Any of the fluxes for soldering will also clean light oxidation. Heavy oxidation -green colored- needs more work. Wire wheel or a flapper wheel will work. Muriatic or phosphoric acids will also do the job, but you want to solder soon after as you can after cleaning.
Bill