Please note: this instructable is written for beginners, and contains lots of very basic information. If you have lots of experience soldering, please use the comments to share your knowledge. Clarifications and corrections are especially welcome.
This Instructable was the lead-off for "How to Solder" month, where we rewarded soldering tutorials with Instructables patches. See the original call for project here at submitting a How to Solder Instructable or just check out all the other soldering Instructables here at the How to Solder group.
Step 1: Tools and materials
For further information and some more technical specs on soldering check out the wikipedia article.
As with many skills, having the right tools for the job effects the quality of the work being done. When it comes to soldering you can end up using a lot of fancy tools, or just a few simple items you can pick up at the hardware store for a couple of bucks. I am going to use a couple of different soldering tools in this Instructable; there are many ways to solder, and you should use what works for you.
At the very minimum you will need the solder and a heat source to melt it - preferably something small which can get to 600-800 degrees Fahrenheit. If you've got that, your ready to make a connection. That being said, there are a wide range of soldering tools and accessories that can be really helpful if you're going to be soldering frequently. Ladyada has compiled a nice list of equipment and sources to buy the tools on her site. I got together a solid supply of soldering tools by raiding the Squid Labs soldering station. Here is the complete list of what I used...
1. Soldering iron
Most people opt for using a soldering iron to solder. It's a great heat source that heats up and cools down quickly and can maintain a pretty constant temperature. Soldering irons can be purchased from a variety of places. I have picked up some at Radioshack - evil yes, but convenient, some from the hardware store, some from garage sales and a bunch more from retailers online. Low wattage (15-40 watt) soldering irons work best for soldering components on circuit boards while more powerful (60-140 watt) soldering irons work well joining thicker materials like braided speaker wire. If you use too powerful of a soldering iron on a circuit board you might damage the components you are trying to join. I like to keep a low-wattage iron around for detail work, and a high-wattage iron that I can use when I am not too concerned about exposing the material I am working with to high temperatures. It's a real pain to solder thick wires without a powerful soldering iron.
The soldering iron in most of the pictures is made by Weller, and has a variable temperature control. This is the best of both worlds since you can set the heat exactly where you want it, but it's significantly more expensive than fixed-temperature irons. If you're just going to do some occasional soldering it isn't a must have by any means. Anyone interested in modding a soldering iron should check out DIY Hot Air Soldering Iron by charper.
There are lots of kinds of solder available. They come in different thicknesses from around .02" to some really thick stuff you would only use on copper pipe with a butane torch. You use thin solder for detailed work like putting resistors onto circuit boards and thicker solder for joining larger materials like speaker wire. I use solder around .025" for most jobs. Most solder is made from a combination of tin and lead - it's about a 60% tin, 40% lead mix depending on what solder your using. Recent international health codes from Japan and the EU (California and New York have adopted similar policies as well) mandate that lead solder be phased out of certain commercial products and substituted with a lead-free alternative. The manufacturing deadline was this past July so we should be seeing the changes now. Even if you don't live in California or New York its still worth staying away from lead solder since lead has been known to cause all kinds of a nasty health effects from birth defects to severe developmental and neurological damage. Plus it's pretty easy to find the lead-free stuff.
Some solder will contain a small amount of silver. This pushes the melting temperature up a bit, but the silver helps the solder to flow and makes a stronger joint. If you are worried about burning whatever your working with, try to stay away from solder with silver in it, but it works very well if you're just joining wires or something that won't be easily damaged. The last thing to know about solder is that you want to use a solder that has a rosin core. The rosin acts as a flux when soldering and helps the connection - it's also the kind that's most readily available at the hardware store and from electronics suppliers.
3. Soldering iron tips
Soldering irons come with a tip, so you don't have to go out and get a special one, but it's important to know the differences between them and make sure you're using the right tip for the kind of soldering you're doing. Some small-wattage irons come with conical pointed tips for detail work, while most high-wattage irons come with a flatter screwdriver-style tip that works well on wires. You want your tip to be a little smaller than whatever you are soldering so you have good control of what you heat up and what you leave alone.
4. Soldering iron holder and cleaning sponge
It's nice to have a safe place to put the soldering iron down in between soldering. A soldering stand safely holds the iron and gives you a place to clean the tip. Some soldering irons come with their own holders. If yours doesn't have one, you can buy one or make one. jaime9999 has a Homemade nearly-free Soldering Iron Stand that is pretty much identical to what you can buy. The stand isn't a necessity for learning how to solder, but it does help.
5. Tools to work with wires
I have a go-to stock of tools that I round up when working with wires or electrical components. They consist of wire cutters, a wire stripper, needle nose pliers, and an automatic wire stripper (courtesy of the Squid Labs soldering station.) The automatic wire stripper is really convenient if you're going to be stripping lots and lots of wire, but by no means necessary. I have stripped lots and lots of speaker wire using my teeth (not the best idea, I know I know.)
6. Clips to hold your work
Often called "third hands" or "helping hands," these little guys help a whole lot when soldering. You have to hold the soldering iron with one hand and the solder wire in the other, so it really helps to have something else to hold the components you're actually trying to join. You can use alligator clips, clamps, or even some tape to hold things in place if you need to. The third hand is generally a good investment if your going to be soldering regularly, and there are plenty of Instructables with with ideas to modify them if you do happen to pick one up. Check out: Make a 3 degree of freedom 'hand' to help with soldering / gluing work and make your 'helping hands' 100x more useful for soldering / gluing small parts by leevonk to start.
If you would like to make a set of helping hands of your own there are already a number of good Instructables on that too. Quick helper for surface-mount soldering by http://www.instructables.com/member/bikeNomad/bikeNomad, QuickMods - Soldering Arms by Aeshir and Build a Pair of Helping Hands by john otto should get things started.
7. Exhaust fan
I do most of my soldering at a soldering station that is equipped with an exhaust fan. It's really not such a good idea to breathe in solder fumes, and soldering does produce fumes. Any kind of ventilation/fan you can rig up will help. Vent the fumes outside or use an indoor fan with a filter if you can't vent them outdoors. Here is a Window-mounted solder fume extractor (not just for RVs!) posted by bikeNomad. Also check out Dr. Solomon's low tech, but functional Solder Fume Extractor if you're looking to build something that you can place right on your table. If you're just doing a quick soldering job, the fumes wont kill you by any means. I have certainly done my fair share of soldering without a vent, but anyone doing repetitive soldering should definitely pick one up or make one.
8. Safety goggles
I hadn't ever used goggles before while soldering, but while doing research for this post I saw it mentioned elsewhere and agree that its a good idea. Little molten bits of solder tend to fly out of the soldering joint when you're feeding in the solder, and if it landed in your eye it wouldn't feel too good.
9. The materials that you want to join together
I was just messing around, and mostly soldering for the purpose of this Instructable so my materials didn't necessarily make anything. You can solder wire, electrical components like resistors and capacitors, circuits, breadboards, electrodes, small pieces of metal and whatever else you can think of. Don't know if it can be soldered? Give it a try - you won't blow anything up.
Once I have got my tools and materials rounded up, I like to pretend that I am a pilot and begin my pre-flight/solder checklist.