For garden-variety wire soldering (like motorcycle wiring harness bits) all you need to do is scratch the wire to a reasonably clean copper. None of that black- or green- corrosion should be there. A simple scratching with a pocket knife usually does the job.
Removable tips? Well, I dunno about that, but my ancient gun just has a piece of solid copper wire bent to fit and crimped at the tip to make a nice tip. Works great.
I use flux-core solder, but I couldn't tell you the reasons why. It's just what "dad taught me..."
The thinner the solder, the easier it seems to be. Real heavy solder takes a ton of heat to melt, and usually that's bad for the wire's insulation anyway. I also use a paste flux on wire joints. It seems to do a good job cleaning right as it's being heated by the gun. Plus, the bubbling hot paste seems to help uniformly heat up the wire joint a little better than a dry joint.
As for what to teach - heat up the wire to the point where the solder melts when it touches the wire, not when the solder touches the gun. The solder will melt and 'suck into' the joint just as mentioned.
Remove the gun and hold the joint steady. As you watch, the solder will suddenly 'flash' cool from the shiny silver of molten metal to a dull silver. When it's dull, the solder is solid and the joint is done.
If the solidified joint looks smooth, like water that solidified to ice, it's a good job. If the solder starts to look lumpy like dribbling candle wax, then the solder probably didn't really stick but rather it's just 'lumped on there...'
Good luck.
Kirk