How to tin a soldering iron after unboxing?
Assuming you mean really, really cheap cheap like the one below, then there is a way out of the mess.

The points on soldering irons like that are simply pieces of copper.  You can file the black stuff off and make the surface smooth again.  Tin the clean, shiny copper surface with solder.

Clean the tip often using a damp sponge.  This means while it is hot and you are using it.  Unplug it if you won't be using it for a while.  That kind of soldering iron is completely unregulated and will get very hot.  Th black gunk will accumulate faster when you just let it sit and heat up.  

My father used to use a bit of electronic solder that had a high silver content to tin the soldering iron, then used normal solder for soldering.  The idea was that the silver solder would prolong the life of the tip.
Don't know if did any good.  When I used the old cheapy soldering irons, I would just tin them with regular solder and file smooth and re-tin as needed.

The cheap things are also handy if you live in the boondocks and can't just buy a replacement tip at need.  They are easy enough to make out of heavy copper wire with a file and a threading die.  For the one in the picture, you wouldn't even need the die - it isn't threaded.