This is for the PC - I think the best program for editing audio is Adobe Audition.
There are so many different approaches to removing unwanted noise. A few easy filters you can apply without too much trouble are the Noise Reduction Category under the Effects Tab. There you have:
Basically, you find your noise floor and or you selectively profile the noise and then apply the filter. You can preview first, in order to see what it sounds like. I believe if the noise is -45 dB or below then you may have a serious problem, which means it may cost more to get rid of than its worth.
If you're looking for more advanced techniques, we can show you but it takes time, the manual and some training - remember this is more of an art, than a science - Good luck!