Typically flux and solder are all that’s needed to “tin” a new soldering iron tip. Occasionally though something more is needed. Although sand paper might do the trick overly enthusiastic application can remove the plating and thus ruin the tip. I prefer to use a product specifically intended for the task such as tip tinner/cleaner from an electrical/electronic/hardware supplier like Radio Shack:
Radio Shack Tip Tinner/Cleaner Compound (0.5 Oz.)
As has been mentioned the work surfaces must be hot enough to melt the solder. The solder should liquefy and flow over the material being soldered. Take your time and practice your technique on scrap material such as copper wire, printed circuit board or thin copper plate. Good solder joints should look bright and shinny. Good Luck.
