The Iron you choose needs to be high enough in wattage so it will easily melt the solder along with being able to reheat quickly to stay hot enough to continue soldering without seizing up on you. A lower wattage Iron like a 60-watt or 80-watt Iron might be great to start learning with, but will be mildly frustrating, as your projects get bigger.
As Solder is introduced to the tip of the Iron the tip cools down because you are introducing a cold metal. If the tip is cooled too much the solder will not flow as well and in some cases the solder will stick to our project.
The Irons that are a 100 watts or bigger are great for doing a lot of work, but your skill level needs to be fairly good because you are going to need to move a little faster. Also at this wattage there are some irons that are internally controlled to maintain certain temperatures such as Weller. Weller has tips with the temperatures marked on the bottom of them such as a number 7 which equals 700F or a number 8 equals 800F and so on.
Should the Iron not have these controlled tips you might also think about getting a rheostat to help control your Iron. A thing to know is that the rheostat does NOT control the temperature of your iron it just slows down the heating process. When you place the rheostat to lets say 7 it heats up slower and when you introduce the solder it takes longer to reheat. So do not use a rheostat on a temperature controlled Iron!
The other issue is what size tip? I like using at least a 3/8-chisel tip because of its size it seems to move the solder better for me. Lots of the lower wattage Irons only have a 3/16 tip and just don’t seem to move the solder as well. Hope this helps you a little bit.