I started my soldering "career" with R/C electric cars and an old soldering iron my father had, and no formal training.
Well that's not quite true, an electronics friend of mine had once told me "solder sticks to whatever is hot", that was my training.
I now hold a diploma as a NASA certified flight soldering technician, and have come a long way. As someone who has done all of the dont's, lemme try to impart some of the lessons I learned the hard way.
First of all there are two kinds of irons-- Temperature-controlled irons, and cheap-o 15/30/45-watt irons you get at RadioShack.
A decent Temp-Controlled iron will run you from $100 to around $150, and is worth EVERY PENNY, because they--
1) Are durable, and made for intense soldering.
2) Heat up instantly (typically takes around 20 seconds to a minute to become hot enough to use)
3) Give you a wide range of temperatures, from doing fine surface-mount work to pipe-melting blast-furnace action
4) usually comes with a very functional stand that gives you a sponge and iron holder.
Now I'm talking about adjustable temperature, not adjustable power, the adjustable-power irons look like temperature-controlled ones but are a lot cheaper, you're not going to get a new adjustable temperature iron for less than $75, but they really will save you a lot of headaches in a short and longrun.
The other kind throttle their power and heat up slowly, using power/cooling balance to maintain temperature.. that means that if you use them to solder with they cool down (from melting the solder) and give very inconsistant performance.
First most important thing you will need is eye protection. I cannot overstate the importance of eye protection, when you melt solder flux will sometimes bubbles off and sends beads of material off. Often if you slip with the iron you will send a blob of melted lead flying, If you solder enough, eventually something hot WILL fly toward your eye.
Second most important thing you will need to solder with is a wet solder-sponge. Really. Any decent iron will come with one, and once you see how useful they are you'll wonder what you did without it.
ALWAYS wet the sponge and ALWAYS use it, just soak it and give it a good squeeze, it should be damp, not wet, and for pete's sake don't wipe an iron off on a dry sponge, you'll just ruin the tip and the sponge.
Thats pretty much it, except for the work and the solder, I'll get into techniques in a moment. First lets go over some do's and don't.
DON'T ever file or wire-brush a solder tip "clean" the surface needs to be very smooth to solder correctly, and there is a coating on most tips you don't want to remove. Flow some solder onto the tip and wipe it off on the sponge, this should clean it to baby's *** smooth.
DON'T store a soldering-iron bare. Always coat the tip in solder before turning it off. This prevents buildup of oxidation. Actually you should coat the tip before putting it down even if you leave it on, but this becomes tiresome, just do it if you are going to leave it for more than a couple of minutes. Unless you work for NASA. don't get me started.
DO always use the sponge EVERY TIME you pull the iron out to use it. yes EVERY TIME. just give it a quick wipe, this will clean off the oxidation that builds up from it just sitting there hot.
DO Clean the tip between joints if you are doing a lot of them. Flux will build up on the tip very quickly, and should be wiped off every 10 joints or so (if you are doing a lot)
Okay a few words on temperature --
A good soldering temperature for most work is around 600 degrees F. I think that works out to be like 315 C. Thats enough to melt the solder and get it to flow around, without melting teh circuit board too.
If you can't get solder to melt or flow properly, don't turn the temperature up! clean the tip and make sure you're using the iron properly.
The only time you should consider turning the temperature up is if you are working on something that transfers heat away very well. Like thick (18 guage or lower) wires, large ground planes, or heat sinks.
Most temp-controlled irons can put out upwards of 800 degrees F, That's hot enough to do anything. but remember.. the more heat, the more likely you will damage solid-state components, and it will cause your tip to become oxidized faster.
In general, run the lowest temperature you can and still do good work.
For electronics/wires always use a flux-core solder. Acid-core is for plumbing, and will corrode wires if you attempt to use it in electronics.
Flux is what makes solder flow. read that again. Without flux, solder is hornery and stubborn. It won't flow where you want it. Often times if you are having trouble with a connection, a tiny dab of fresh solder will smooth it out, because of the flux inside it.
But Flux is a double-edged sword. It contaminates your tip, and needs to be cleaned off at regular intervals.
Flux is also available separately, its a sticky-gooey mess and I've never gotten any use out of it. There are some cases in flight-work or complex re-working of delicate components where it's useful, but for day to day stuff don't worry, the flux in the solder is plenty.
Start with a clean tip, a few quick swipes on the sponge.
But remeber this, a bare iron is practically useless. You need to put a dab of solder on it to get any work done. It's called a 'solder bridge' and is what is used to transfer the heat from the tip to the work.
So dab a bit of solder on the tip, and make sure the solder-dimple it touching the work. Now poke some solder in there (touching the work right next to the iron, not the iron itself) and it shoudl flow into the joint. Remove the iron and viola, perfect.
It is often necessary to "reflow" a joint. It's important to dap a bit of solder into a join you are trying to get to reflow, so it has some fresh flux.
Some chips are more robust than others when it comes to being soldered, some chip snad components can be easily fried by using too much heat. The rule of thumb is 5 seconds at 600F.
Consult the data sheets if you are unsure, but most components are rated for 5 seconds of contact with an iron before being damaged. Most chips will take a little more but keep mental track, from the time you touch it, count to 5 and remove the iron NO MATTER WHAT. You can try to reflow it in a minute when it cools down.
Try not to solder a chip from consectuive pins, or all at once. Skip around like a torque sequence. And give the chip a few seconds to cool between joints. An ounce of prevention.
How to tell a good joint from a bad one--
In general, if it's clean and shiny, it's a good joint. If it's grainy or irregular, it may be a cold joint.
A Cold Solder joint is caused when the work moves while the solder is cooling, this makes a very weak joint that will fail eventually, if not right away. usually a quick re-flow fixes this problem.
Avoid an excess of solder, if too much solder is on a joint just touch the iron to it and let it soak some up, then wipe it off on your sponge (you DO have a sponger, right?
If you have way to omuch solder, or you need to unsolder a joint, consider getting a solder-sucker. They are cheap and when you need one, nothing else really does the job.