Hi I have auto. Irrigation sistem but die computers have to start a pump the output voltage of the computer is 24vac so I have got a 24vac contactor and only coneccted the live wire from my computer and the other point of my contactor that's marked C I have hooked up to the neutral of my 220vac (home power) but it doesn't work why?

You can't get 220/240 volts off the neutral.
Are you saying your computer outputs 24 volts ac?
Or your computer power supply?
You should be able to apply 240 volts to the transformer primary, and have the secondary 24 volts go to the contactor. The transformer needs to be large enough for the contactor coil, and the contacts need to be rated for your load(pump).

Where are you located?

When I saw the 220, I thought they may not be from around here?

The computer output is 24v ac, you see I have 24vac from my computer that supplys a commen port(neutral) and my station 1(between commen and my station 1 I get 24vac).But the contactor and the motor are a long distance from my computer so I wire only one wire and at the motor I picked up a neutral. The I should be able to get 24vac?

You don't say the amount of current the 24 Volts out of the computer has.
See if the contactor energizes when connected to the computer and watch the voltage as you connect. If the voltage drops significantly, and doesn't energize, you need a more sensitive relay(less coil current).
Your pump and computer wiring will have NO connections between them.
Computer only connects to the contactor coil. And your pump only connects to the contactor contacts and your source of power(220).

Thanks for all your answers the main problem was that I tried to take the live wire from my computer that supply's 24v between conmen and station 1, I only took the station 1 wire and picked up a commen at another cb-bord(from the neutral bar). The problem is when you take 220 and convert it to 24v it becomes positive and negative and it's like taking 2 car batteries and take a positive from the one and a negative from the other one... it would just not work!!

A Transformer increases, decreases, or the mantains the same voltage(isolation), It doesn't convert to DC. Rectifiers will convert to DC. You have more than a transformer.
When you say computer, I take it you mean the Sprinkler Timer/Controller?
Is the Pump to run as each zone is active, Can you tell more what you are wanting to do?