when you tin an iron tip (or a piece of wire for that matter) you are covering it with a clean new film of solder. if the tip is in good condition, just wipe it on a damp cloth when its hot (im a slob, i just you use my shorts while wearing them!) and then feed a bit of solder on it. the idea being 1) the tip wont oxidize as quickly
2) heat is transferred more efficiently.
If the tip is badly corroded, you will need to file the tip back to its original shape first (normally conical or wedge shaped). File away all pits in its surface. Then tin as above.
have you ever watched a movie where a pilot talks down a passenger flying a plane? not the best way to learn!! same here. i suggest busting open an old radio or something and start practicing on different aspects of soldering/desoldering. desolder caps and resistors and solder bits of wire back in their place etc.
dangerous but highly effective tip # 1: when i was an apprentice auto electrician we often had to desolder much larger things than a circuit board hole. for instance rectifiers, field windings, solenoid caps etc. what did we do? as soon as we took the iron off the part, we blasted the molten solder with an air gun! utterly clean job. if you were to do this at home, wear goggles and dont aim it at anything/one you dont want coated in a film of hot lead! its not too bad on skin but eyes would of course be badly damaged.