How do i tin my soldering iron and question?

ok so i just got a soldering iron from radioshack. its is adjustable from 20 to 50 watts. I also have leadfree solder from radioshack. so without tinning or cleaning of any sort (im new to this) i made a couple connections. (8? not at the same time) And so i looked up how to solder cause knew how to do it but i wanted to be more informed. So i learn you have to tin the tip, i go to attepmt to do it and my solder was "beading" or "balling" away from the iron and it started to turn the tip black and i attempted to rub the black of with a sponge but nothing. so what do i do? I also may have accidentally tinned it the first time because i know i accidentally touched the iron with the solder and now there is a spot that is shiny and soldering is easier when i do it with that part. DId i accidentally tin it or is it something else?

Rub VERY gently on some wire wool.