Today we are transitioning from the old way of doing things, where we used tin/lead solder, to new lead free solders. The old tin/lead solder is still widely available for hobby use, and has the advantage of a lower working temperature, but some lead will leach out when products using this type are disposed of in land fills. That is why most commercial soldering is done with lead free solder.
It is up to you to decide which type of solder to use in kit building. I prefer to use 60/40 tin/lead solder because of the lower working temperature. I don't produce enough electronics that will end up in the landfill for the pollution to be much of a worry. The new lead free solder is better for the environment, but the higher temperatures required make it more likely that you will damage a component soldering it in, or damage the PC board you are working on. As in much of life, there are trade offs to consider in selecting solder.
Never use acid core solder for electronic use, use only rosin core solder. I like to use solder with a diameter of about .032 inches. This is fairly fine solder, but I find it easier to control the amount of solder in a joint with the fine diameter solder.
I started out using a cheap copper tip soldering iron. The big advantage of this type of soldering iron is that it is inexpensive. The disadvantages are that you cannot lower the temperature of the iron, and the copper tip erodes away with use, and fairly quickly at that. Often the tip is shot after building a single kit. These soldering irons can be had for under $10.
A much better choice is a temperature controlled soldering station with an iron clad tip. These units allow you to dial in the working temperature to minimize the possibility of damage to the PC board or the components. The iron clad tips will last a long time. The station I am currently using has had the same tip in it for about five years, during which time I have built several kits and hundreds of scratch-built circuits. The big drawback with these are cost. They usually run anywhere from $30 to over $100 for basic units, and commercial grade units can cost several hundred dollars. I have found really inexpensive soldering stations on sale for as low as $10.
There are two basic types of kits on the market today. Most kits use through hole components that have been around for many decades. These parts are large enough to see with the naked eye, even for us older folks. Commercial products today use mostly surface mount components which are very small, leadless components that solder to pads on the PC board. A typical surface mount component is 2 millimeters by 4 millimeters, or roughly the size of a grain of rice. Working with these components can be tricky.
Here is a link to a photo of a through hole board. Through hole picture
Here is a link to a photo of a surface mount board. Surface mount picture
The main reason to be concerned about the type of kit, through hole or surface mount, is that they require different size tips. For surface mount, you want a very fine tip to keep from bridging several connections at once when working with surface mount integrated circuits. With a very fine conical point, you might not be able to transfer enough heat to a through hole connection to get a decent connection. For through hole kits, you want a slightly larger tip.
The secret to a good solder joint is to transfer the proper amount of heat from the soldering iron to the two pieces being soldered in a reasonably short time. There are several things that will have a bearing on how much heat and how long a time. In general, the smaller the soldering iron tip, the longer it takes to transfer a given amount of heat. The size of the things being soldered are also a factor. Bigger things, like a ground connection, or a connection to a variable resistor, for example, take longer to reach the temperature required to melt solder than a regular connection. The larger the mass of metal you are heating, the longer it takes to get it up to temperature.
Another factor is the temperature of the soldering iron tip. The lower the soldering iron tip temperature, the longer it takes to get the solder joint up to working temperature.
You might think at this point the solution is to use a big point at high temperature. There are several problems with this approach. A big tip is likely to cause a solder bridge, that is is, to leave a small finger of solder between one connection and another connection. The tip has to be sized to the spacing of connections on the board you are working on. Through hole boards often have 0.1 inch or 0.05 inch spacing, so a regular conical tip can be used. Surface mount boards have much tighter spacing and require the use of a fine or very fine conical tip.
The problems with using too high a tip temperature are manifold. First off, at high temperatures you will burn off the rosin before it has had a chance to clean the connection. This can lead to poor solder joints with a dull appearance and small black flecks in them. Another problem with high temperatures is that it can damage components. Many components are rated for exposure to 300 degrees C for ten seconds during soldering. Poor soldering techniques can exceed these ratings and cause the components to change value or have their parameters degrade. Another problem you can run into is lifting a pad. If you get a pad on a PC board too hot, it can delaminate from the board. Repairing a PC board is an expensive, involved process, usually not worth it in a hobby kit.
When you first fire up your new soldering iron, you will have to get an initial coating of solder on the tip. This is best done with Tip Tinner and Cleaner. They sell it at Radio Shack for about $7 for a small tin, but that tin should last several kits. The Radio Shack part number is 64-020. It is well worth the money. Leave your soldering iron turned on until it comes up to temperature. This will take 3 to 5 minutes for a cheap soldering iron, and about a minute for a nice temperature controlled soldering station. After it is up to temperature, stick the tip all the way down through the Tip Tinner and Cleaner paste, it may sink slowly, then withdraw it. The tip should have a nice shiny coat of solder on it.
With 60/40 tin/lead solder, I like to set the soldering iron to 300 degrees C and it should take less than five seconds to make a typical PC board connection. Wipe your iron clean on a damp sponge, and apply the tinest amount of solder to the tip so that the tip is nice and shiny. Position the tip so that it presses against both items that are being soldered, the component lead and the PC board pad for example. Allow a couple of seconds for the joint to heat up, then apply a bit of solder to the joint. The solder should flow into the joint and wick across the PC pad and up the component lead. Remove the soldering iron and allow the solder joint to cool, then trim the excess component lead off.
With lead free solder, you have to jack the soldering iron temperature up. I have had friends tell me that they have had to go to 320 or even 350 degrees C to quickly melt lead free solder. With 60/40 tin/lead you can run as low as 270 degrees C. While the 350 degrees C seems dangerous at first from a component exposure stand point, keep in mind that there is a temperature gradient from the solder joint to the actual component body. As long as you don't stay on the solder joint too long at 350 degrees C, you should be okay.
First wipe your iron clean on a wet sponge. Next melt the smallest amount of solder possible on the tip of your iron. Touch this spot to the PC board pad and the wire lead sticking through the pad and hold the iron in place for three seconds to give everything a chance to heat up. After three seconds, touch the solder to the PC board pad. The solder should melt and wick across the PC board pad and up the wire lead. Remove the solder, and then remove the iron.
If you don't get good flow the first time, just touch the iron to the solder joint for three to five seconds. The solder should wick across the PC board pad and the wire lead. You may have to apply a very small amount of solder the second time to get good flow. This is because the solder has flux in the center that cleans impurities off the solder joint and facilitates good solder flow.
I have seen beginners take the soldering iron off before the solder is removed. This results in the solder being stuck to the PC board pad. Resist the temptation to tug at the solder to try and free it, this will not work, and may lift the pad from the board. Instead, just apply the soldering iron to the joint until the solder melts, then remove the solder, then the iron.
If you end up with solder on only one half of the PC board pad, try placing the small amount of solder a little farther from the tip, and lay the iron sideways against the PC board pad half that doesn't have solder on it. (With the solder on the iron in contact with the PC board pad.)
Here is a link to a photo of an iron tip soldering iron correctly positioned to make a good solder joint between an IC socket pin and a PC board pad. Soldering picture
Here is a link to a photo of the IC socket after it has been soldered in place and some components have been added to the back side of the board. (This is the AR2N6 receiver kit from Elenco Electronics, Inc.) Socket picture