how do i get rid of mold on walls without making a mess?

How big is the patch? If it's a few spots, a cloth with diluted washing up liquid or a mould spray, wipe once, fold the cloth to a clean side, wipe again. Don't dry brush it, that's what puts spores everywhere and makes the mess. Mask and gloves, window open. It will be back in a month unless you sort out why it's there. Black mould on walls is nearly always condensation - cold corner, furniture pushed against an outside wall, washing dried indoors. Pull the wardrobe a few inches off the wall and get some air moving. If it keeps returning on the same patch or the plaster feels soft, that's damp coming through and no cleaner fixes it.