Hello I've just started programming with arduino so don't kill me 
I began reading some examle project and I noticed that, sometimes, just connecting cables is not enough.
For example, I wanted to connect my playstation 2 gamepad to arduino, but I read that in this case another power source is required.
I wanted to connect an external actuator and I discovered I needed an adpter.
Even for a led I discovered you need a resistor between arduino and led itself.
On the contrary, for nintendo wii's nunchuck no other device is required and just cables are ok.
My question is: how can I see if "something" is required or not?
It's ok to have tutorials but what if a day I wanted to do something it is not on a web page?So I want to know some "best practice"s, what do I have to look in order to think "ok here I need a resistor/power source/ecc"...can you help me out?
Strangely enough I know the theory about this, so if you talk me about voltages, resistance, ecc. I can understand you perfectly...problem is I don't know how to apply theory to pratctice T_T
Hi,
Often the mechanical / cabling stuff is a hassle. Take a look at this about cabling and connections:
http://arduino-info.wikispaces.com/Cables
Things can also be a lot more plug-and-play with a Sensor shield:
http://arduino-info.wikispaces.com/SensorShield
And some people like to use these functional "Electronic Bricks", especially for quick prototypes:
http://arduino-info.wikispaces.com/ElectronicBrickStarterSet
DISCLAIMER: Mentioned stuff that is available my own shop... and other places
My question is: how can I see if "something" is required or not?
By reading the device's data sheet. That will tell you all that is required to use the device.
Oh, that and a basic understanding of electronics - things like Ohm's Law, Kerchoff's Laws, Thevanin's Theorem, First Order circuits, etc...
Yep, the datasheet is key. All you have to do is find it for whatever device you're interfacing to and figure out if you're going to need a seperate power supply (since Arduinos can only supply so much current at a certain voltage). Or if you communicating with it, you might need some type of level shifter between the microcontroller and the device to be able to talk to it. It's all in the datasheet for what you're connecting to. It's good that you understand circuit components and the theory behind them because this alone can take you a long way towords building your own circuits. Good luck!
And in the end experience is the best source of knowledge... So Jump In and Have Fun, I do and I am a 66 year Auld Retired Phart and I enjoy every moment of my experiences with Arduino and electronics... Even If I do talk too much. IMO and others I am sure...
Doc
My question is: how can I see if "something" is required or not?
A good clue is if it has a socket on it.
I am a 66 year Auld Retired Phart
Hey, I'm even older than Doc!
And I still spell bettern you.... 
The most common reason you need "something else" is when the arduino pin either supplies too much current or not enough current. You look at the specifications or a datasheet to find out the right amount of current.
For example, on http://arduino.cc/en/Main/ArduinoBoardUno it says DC current per Arduino Uno I/O pin is 40 mA. A typical robot motor might require 1 Amp, so you need something else to "step up" the current going to the motor. On the other hand, a typical LED can only handle around 20 mA of current, so you need to add a resistor to "slow down" the current through the LED.
Often the maximum is specified as maximum power dissipation instead of a maximum current. DC power dissipation is (input voltage - output voltage) * current. Let's say you have a 5 Volt linear voltage regulator rated at 1 Watt. That means if the input to the regulator is 6V, you can run 1 A of current: (6-5)*1 = 1 W, but if you connect it to a 9 volt battery, you can only run 250 mA: (9-5)*0.250 = 1W.
Note that when you get close to the maximum, components can get very hot, so you have to start thinking about heat sinks and thermal design as well. In general, you want to stay well away from the maximum ratings. Also, a board may be designed to use less power than the chip's maximum, so you should keep that in mind. For example, Arduinos have stricter limitations when powered by USB.
Another common reason you might need "something else" is incompatible voltages, even though the current might be okay. For example, you can't hook up an arduino directly to a computer's com port because the voltages are different. Also, to save power some chips run on voltages much less than 5V. This is the case with your playstation gamepad. It runs on 3.3V versus 5V for the arduino.
When in doubt, ask someone, because each component has unique characteristics. For example, when connecting audio equipment, you have to consider things like impedance and isolation. Inductive loads require flyback protection diodes. Weak signal sensors require amplification. Noisy signals require filtering. There's no one size fits all solution to determining if you need extra parts.
/* Ya, Ya, Ya... soz U sa...
Older than ME... OK but today everything hurtz, I feel older than dirt. Happens sometimes. Some are Definitely bettern otherz.
*/
Doc