syrine:
for my joystick a got 6 wires.
and the other to wires are ground and source.
so i created a circuit where i create voltage divider with 2 resistor of 2,2 kohm and 2 resistors of 2,8 kohm .but it didn't work .i don't know where is the problem
can you please help me 
Without seeing a picture of the -bare- joystick (and even that might not help) - it is almost impossible to tell you what would work or how to get it to work. Heck, your experimentation may have killed the wheelchair controller - or your Arduino output pins; who knows...
At first - it sounded like the wheelchair joystick was simply a couple of potentiometers; some of the older chairs used these (btw - what make and model is the chair?); newer chairs tend to use inductive coil systems or hall-effect sensing joysticks (more reliable than potentiometers - which is a big thing when you are dealing with such a chair for a disabled person!).
But you mentioned two other wires for power and ground (8 wires?) - unless I am misunderstanding you (maybe due to the language barrier - though your english is better than my french!). Now - if it is truely only 6 wires (ie - four wires plus power and ground) - then that could mean something different. Again, information, pictures, etc of the chair and controller can be essential.
Wheelchair controllers are very finicky, for a very good reason; they basically refuse to work unless the input parameters (and other things) are perfectly in sync and specification; if anything is slightly "off" - the controller will refuse to do anything. This is meant as a safety feature, to prevent a chair from lurching off at high speed with a person in it, unable to control it. So - you may have had everything correct voltage-wise, but the controller found that the impedance of your circuit was off, or that the voltage was slightly wrong in some manner, or the signals weren't perfectly in sync, etc - and said "no way, no how" - and refused the commands.
Honestly - the easiest and best way to control wheelchair motors isn't with the wheelchair's controller (which is of a proprietary design that is nearly impossible to get information about how to interface with it) - but with a 3rd party high-current DC motor controller, such as one from Vantec or Roboteq - just be prepared to spend a boatload of cash - but they are heavy duty devices, and will work with a variety of input signals (both companies make controllers that can take servo input signals for control).
Lastly - be careful with testing your code on the wheelchair base; they are heavy and powerful things, and can seriously injure you if don't take good precautions. There was one individual in these forums who mentioned they had one go "runaway" and careen into a wall; had that been a person, serious injury could have resulted. So - test your code completely with the base up on a stand or something, before trying it out with the wheels on the ground - and always have some kind of kill switch setup on the system (an on-board kill switch and a remote kill-switch, combined together, are ideal).
I speak from experience on this; I work with a gentleman who has a non-profit doing repair and refurbishment of mobility chairs and scooters for donation. We also build remote-controller are robots, custom art chairs, and other such devices using a variety of parts (both from old mobility chairs we can't donate, or from other parts we scavenge or purchase). We've come to respect the power of the motors and weight of the chairs. So be careful.