A power supply is a power supply. It has positive and negative. The "COM" refers to the common negative when the supply has multiple positive output voltages (and may also have one or more output voltages also negative to the common as some PC power supplies do or used to)!
If it has only one output it should be marked positive and negative. Just assume "COM" means negative. On microcontroller modules the negative is generally called "GND" (ground). Not the same as the "Earth" symbol on a power supply which relates instead to the power cord.
CrossRoads:
Your schematics in reply #14 might be okay, they are pretty small to really be sure.
The problem with #14 is that it shows wires randomly going from place to place. This is not how you wire things to actually work, and work reliably as so often comes to questions on these forums. As I would think you realise.
I explained, detail by detail in #9, exactly where wires should run and how they should be grouped.
Your diagram in #16 is more appropriate, but still shows an "aimless wander" point in the middle. The strip itself is the point where the cables must converge and the apparent idea of the two grounds is that one connects to the 12 V power supply and the other goes to the Arduino with the data wire. They are not there just to share the current; you do not need to connect them together! You can connect the 12 V wire to power the Arduino also to the adjacent 12 V terminal on the strip. All this I explained in #9.
Yes Arduino has many resistors. No, Arduino cannot take much power. You do not know much about electricity, as you said before. That is why you ask for advice. Your knowledge of the uses of resistors is equally limited.
You may have seen schematics elsewhere on the internet where a resistor is not used in the data line. Everyone is allowed to publish whatever they want on the internet, there is no qualification required in order to do that. So there are many bad schematics out there. They work for the original publisher by luck. Others copying those schematics may not be lucky. Those others come here for advice and the experienced forum members can help them with better advice. But they cannot delete the incorrect schematics from the internet. They wish they could, of course!
The same 12V that goes to the strip can go to Vin.
If your power supply has 5V and 12V outputs, then connect 5V to 5V on the Arduino, and disconnect it when you are connected to a PC via USB; many PCs don't like it when 5V is connected to the USB's 5V, and may shut down the port.
When you are done downloading code, you can reconnect the Power supply 5V.
The same GND that goes to the strip goes to the Arduino Gnd.
470 ohm.
If the strip is located near the Arduino it may not be needed.
For example, I have 4 rings of 43 WS2812B in this video, with several feet of wire between the Arduino controlling them. No resistors were used.
1/6, 1/8, 1/10, 1/4, it is not critical, there is so little input current flow, just 0.000001A. (1uA)
W = I^2 x R
.000001 x .000001 x 470 = 0.47nanoWatt
1/4 will likely be easier to use, with larger diameter leads.
on a power supply which relates instead to the power cord.
You can connect the 12 V wire to power the Arduino also to the adjacent 12 V terminal on the strip. All this I explained in 
