Tapping fixing it is a sure sign of bad joints. Often the fault will be visible to the naked eye as a crack all the way around the pin. It will often arc a bit when the connection opens up, and eventually this burns away more solder and it gets bad enough that tapping it won't fix it. Not only that, but intermittent connections can and do blow other parts, so I always advise to stop using equipment immediately to prevent further damage.

Know anyone nearby who has any soldering experience? Is there a local hackerspace or similar geeky meeting place? Failing that, look on youtube for soldering tutorials, and as has been suggested, practice on something that's already junk.

Unplug the machine and give it a few minutes for everything to discharge, it doesn't normally take very long. You'll need rosin core solder and a soldering iron. Look for the standard tin/lead stuff if you can find it, the lead-free solder is harder to work with and tends to make lousy joints. Hold the iron to the joint until the existing solder starts to melt, then feed in a bit of fresh solder, let it flow for a couple seconds and then take the iron off it. You should end up with a nice silver colored joint that is slightly concave. If you have a bulging ball of solder, you've applied too much.