I have a outside unit,it is blowing hot air into home threw vent but if I put the cool on its doing nothing there is no cold air coming out of my vents, the fan on the top of the unit is running when I turn it on, so I was wondering if this is something simple to take care of, for the reason its not blowing cold air threw the vents , but it dose when we put on the heat.

I guess this is a heat pump since you did not say. If it is to cold you are not to run the A/C any way.

If heat pump it could be reversing valve.

It could be the coil for reversing valve.

It could be the change over relay.

Or a few other things.