Need to refill my car A/C. Help?

So recently I just started driving my 2002 Ford Expedition again after it being parked for almost 3 years. Needless to say I've spent serious cash catching up on maintenance and repairs. For example I just replaced the throttle body and vacuum, got new spark plugs and fuel injectors, got new tires and changed the oil and filter. All this I've done myself with the help of a few friends. Well, the Exp is running better than ever and I'm excited to start driving it again since I just now was able to pass the smog test and am heading to the DMV to get my tags. Now the only thing left to do is make the A/C blow cold again. My friend bought the refill tanks and gauge. The only problem is some guy at autozone said I might need to get the A/C tank flushed or something like that because I explained to him that since I bought it back in 2009 from my parents who were the second owners that the A/C never blew cold. I've never heard of flushing the A/C unit and need to know if I can just go ahead and refill it or do I actually need to get it flushed professionally? That's my question thanks in advance guys.

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