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Not knowing your soldering experience:
Remember that the items being soldered must be hot enough for solder to flow on them & that the iron is to heat those, not to directly melt the solder.
A tiny amount of flux cored solder on the iron first helps thermal contact to get heat into the items being joined.
Hold the iron tip in the angle between the PCB pad and pin.
After a second, apply cored solder to the pin/pad - it should melt easily, without needing to touch the iron tip.
If it does not flow properly and just blobs, lift the blob with the iron and wipe or flick it off the tip.
The solder should have left a layer of flux on the joint parts, which will help remove any slight surface oxidation or tarnish in the next attempt.
Just try it again.
If the parts are obviously tarnished, clean them first - but that is quite a rare thing to have to do.
Flux is just a chemical way of removing tarnish from the parts being soldered. Most people do not have rosin based flux in their toolbox so using physical methods to brighten up the copper or tinned component leads will let most people get their part soldered today with the tools at hand instead of running out to Radio Shack to get s pot of flux. Oh wait, have to wait a couple days for amazon to deliver it - no more radio shack.
I'm with rjenkinsgb on this one - rosin-core solder is really the best way to go for a hobbyist learning to solder. It's very easy to use and just works - no need to apply external flux, which means you don't have to go through the process of finding the best flux to match your application and the solder you plan to use. Just buy a spool of 0.015" rosin-core solder and that'll work for a wide variety of electronics projects.
I wonder if "rosin" when referring to solder flux cores is like saying "pencil lead". It used to be made out of a certain material but due to health and safety reasons, it was changed to something else. Regardless, a lot of people still call it by the old name.