"Tin" the end of the soldering iron (or gun) gun first-

Sand it lightly to clean it, then turn it on, and after it's fully up to temp. touch the solder to it. Just enough so it melts on there. You can wipe off the extra with a cotton rag if you get too much on there. Now it's tinned, and will work much better.

Now, when you are ready to solder, let the iron heat up fully (5-6 minutes should do it) now touch just the slightest amount of solder to the tip, it will get shiny. NOW touch the area to be soldered for a few seconds making sure you have a desent amount of contact with the part. Next touch the solder to the point where the IRON TIP meets the WORK, and it should flow right onto where you need it.
Don't add too much solder, just 'til it looks shiny, then remove the solder, then the iron. Don't move anything you soldered for a couple of seconds.
A good solder joint will look shiny, not dull or pitted.