Get some steel wool and scrape any gunk off your tip, then get a tip tinner from radio shack, dip the tip of the iron while hot into the tinner, then turn off the iron and leave that blob of tin hanging on the tip... Let it cool. Fire your iron up again, this time, wipe the excess tin blob that you left on a wet sponge... Your tip should be tinned. If not, repeat these steps a few more time (skipping the steel wool scraping if repeating)...
Alternatively, if you don't have access to a tinner, flow a blob of solder on the tip, leave the blob on, let your iron cool off...
Also, a tip for everyone who solders, it's always a good idea to leave a blob of tin before powering your iron off, it seals the tip from oxidation while not in use, and while the tip cools, it'll hit the critical temperature for a eutectic bond between tip and tin...
I have been soldering for 25+ years...
I highly recommend NOT using any No-Clean flux and/or Solder. If No-Clean type of flux isn't properly gassed, it will corrode and eat your metals, I recommend rosin-based soldering of anywhere that doesn't involve any inhalation path during vaping off your DIY vape mod...