My radiator fan runs constantly?

I have a 2000 ford windstar and the fan now runs constantly. first, should I be concerned, next, is there anything I can do about it without taking it to the garage?
Update: as bad as this may sound, I am not very familiar with the ins and outs of the vehicles. So when I am told to check the realys, I have no idea where to begin. I am sure I could figure it out with a little guidance.
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