Good grief what a lot of posts, all about anything but what the OP asked as
far as I can see.

To tin a soldering iron get a wet sponge.

Let the solder tip reach full temperature. They make little trays with
sponges in them just for ths purpose at Radio Shack & other electrical
supplies & box stores.

Wipe the hot tip on the sponge, moving it fast enough to not burn the sponge
(the water makes it easy) & dislodge the oxidation.

If a wet sponge doesn't work well enough, it might be necessary to gently
scrape the tip with a knife or any blade, held at right angles to the tip.
Then a few swipes across the sponge again on the tip, and it should take a
tin nicely.

It's in "good shape" when resin-core solder covers the tip in a nice,
usually shiny coat of solder. Shake off the excess solder. Done. It'll do a
much better job of transferring heat to whatever you are soldering if the
tip is clean and tinned.
Many people get the little tray with sponge and set it near the work so
if/when oxidation starts, a wipe or two on the sponge will renew the tinning
quickly.

Ths same informatioin is available many places on the web; especially
soldering tutorials.

HTH,

Twayne`