You need a damp (not wet) soldering sponge to wipe your tip clean on.
If you haven't got one and cannot find a store to buy one, ask a person of the female persuasion for a make up removal sponge (it's the same yellowish material and works just as well... That is, if she doesn't use other means like cotton wool pads). They are round, but can be cut to size with scissors, if you have a holder - or you could use a metal lid from a can of proper dimensions.
Apart from that, you could use some tip cleaning paste. It's a flux + solder paste in a small can that you dip your tip in a few times. It smokes a bit while cleaning and after a short time, clean it off on the aforementioned damp sponge - repeat until your tip is like silver and easily wetable.
Never try to scrape or file residue off a tip, it will destroy it in no time, as the tip is copper with a thin plating of iron - if you file through this very thin layer, the copper will be eaten hollow in a single soldering session and you will have to file it again to use it... You can't do that all that many times.