Soldering iron tip turns black after first use, and won't melt solder
First, go read up on soldering iron care.  You clearly didn't keep your tip tinned.

Your soldering iron almost certainly came with some sort of small sponge.  This is meant to be moistened.  After each use, you wipe the tip against this moistened sponge.  That clears off flux residue, and some of the oxidized solder.

When just wiping doesn't restore the tip to shiny condition, you have to add a little solder to re-coat the tip, then wipe.  This is called "tinning" the iron.  Keeping the tip tinned makes the thin layer of solder oxidize, not the iron coating of the tip itself.

Even if you are careful, black spots of oxide can still accumulate that don't come of with wiping.  There are tip cleaners for this purpose.  If you have good soldering intuition and know what you're doing, you can very gently scrape the black oxide patches off the tip with a small screw driver, then tin to restore to good condition.  The reason this needs to be done gently is to avoid hurting the iron coating.  Once that's gone, the tip is trash.

I've seen brass spiral mesh used for the purpose of gently scraping tips, but don't have any personal experience with that.

Despite all the above, if your soldering iron isn't temperature controller, you will have problems.  The tip will be way too hot most of the time, and whatever is coating the tip will rapidly oxidize.  Fixed-power irons are much more difficult to maintain and to use properly.  Their only purpose is to appeal to hobbyists that buy on price.  They are not real soldering irons.

If you plan on soldering more than once or twice, get a real soldering iron.  Last time I looked, the Weller WES51 was the lowest price reasonable iron out there.  You can get much fancier, and pay much more, but the WES51 is still good enough for even basic professional work.