The iron used to solder must be of a high enough wattage to readily melt the solder and be able to reheat fast enough to maintain the necessary melting temperature. The tip can't be so small it can't maintain the heat nor so big it covers more area than wanted.
For example a 75 or 80 watt iron is sufficient to begin soldering with, but it will continue to get hotter, as it has no temperature control. An iron of this type should be used with a rheostat in order to prevent overheating while it is idling. You should be aware that it will eventually reach its maximum temperature, so cannot be left for long.
Most temperature controlled irons seem to be produced in 100 watts or more. These internally temperature controlled irons maintain a constant temperature. These are normally supplied with a 700F° bit (number 7) and are sufficient to melt the solder without long recovery times. You can obtain bits of different temperature ratings, commonly 800F° and 600F°. You can also use several sizes of tips for different detail of work.