Adding to Jasljs's good post: The wattage has to be appropriate for the job. If there isn't enough, you will get a cold solder joint. Too much and applying to much heat (contacting for too long) and you start melting things other than solder. The lowest wattage irons are only good for component replacement, but won't nearly be enough heat for the speaker binding posts. For general purpose use, a spade tip, like a common screwdriver, is best. A pointy conical tip will drive you crazy with inadequate heat transfer most of the time, unless doing very fine work. And don't move until the solder solidifies. Blowing helps.
