Need to refill my car A/C

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.

Use the refill and if it still doesn't work, then go to a professional. Trying the refill is much cheaper than having a mechanic to just inspect it.

Refill

First try adding refrigerant to see if it's just low on charge. If that works, you're fine. If it doesn't, then follow these AC trouble shooting guides

http://ricksfreeautorepairadvice.com/fix-ac-in-car/
http://ricksfreeautorepairadvice.com/find-ac-leak/

There's no question on this. Return those cans and have a professional do the AC.
The system needs to have all the o-rings replaced and hoses inspected. The shop will then pressure test the system.
They will try to manually turn the compressor and if rusted shut soak it in oil and let it sit and then see they can get it working.
They need to inspect and test the clutch plate, and lube.
Test the electrical connections.
Then after all that they will need to pull a vacuum to fully dry the system, that may take hours.
Then fill the system from vacuum state, and put in the correct amount of AC oil.
The cans are OK on an older car that the AC is working, but not so cold, then OK top off the coolant.
You can see you have a number of issues, and any short cuts might work, but could ruin the system in a week or two.

Once a car gets older a slow leak is the typical problem. Usually filling some will be good for the season a huge bargain as compared to anything else.

You don't "flush", you evacuate. With a vacuum pump. Then you'll see that the system leaks. Then you need to locate and repair the leak, and evacuate again.

When the system holds the vacuum, you can charge. But you need a set of a/c gauges, and must understand how to properly charge an evacuated a/c system. Only a trained person should be doing it.