Hey so earlier today I took apart an old RC helicopter's controller and scavenged a few parts, including an analog stick. I spent some time trying to figure out which were the x and y sígnals plus the power and ground cables and how to plug it to the arduino, but didn't have any luck 
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So I figured someone here could actually figure it out and tell me how to plug everything up to the Arduino.
So if anyone knows something about joysticks I would truly appreciate your help. Thanks!
Images inline to save others downloading
Looks like plain old potentiometers to me. Are you getting a varying voltage on the X and Y as you move the stick? If you put 0 and 5 on the power, you will get varying 0-5 on the outputs.
So just wire it like 2 potentiometers.
the two blue things on either side of the stick will be potentiometer sliders. with the stick in it's neutral position, each of the two potentiometers (or pots for short) should give a reading roughly misway through their range. Many pc joysticks years ago used a similar arraingment.
with almost any multimeter you should be able to trace the wires to figure out which contact pin goes/does. there should be two contacts common to both which would be your vcc(power) and Gnd (ground or 0v). the contacts that are not common should be connected to the central pins on each and are the analog version of the data line comming out (voltage on these pins will vary depending on the knob position.
having another look at your photos, zooming in on the underside of the board, you have 1 common wire that goes to all three pots. on each of the pots the middle terminal is tied to the third terminal and then back to the cable header. the variation of resistance should cause the voltage to vary. Put the 5v onto the common pin on the header and the other three go to your analog pins on the arduino.