How to fix leaking valve

In the process of putting in a new water heater, I shut off the cold water at a point much farther from the water heater than its own cold water feed valve (there seemed to still be a steady trickle through the one right above the heater, no matter how tightly I turned it off).
The one far away was VERY hard to turn at first (has a lot of that greenish-white corrosion around the handle, too), but it eventually worked. however, now there's a steady drip coming out from where the stem goes into the packing nut (I THINK I have the terminology right here; the drip is coming out along that little post that the wheel turns).
So am I correct that my first course of action is shutting off the main water to the house (THAT valve is about 5 feet to the left, after a few elbows, and it is also pretty caked up with green and white gunk, so I hope to hell I don't run into the same issue there), removing the wheel, and looking at that stuff in there? I've looked at the anatomy of valves, and can't see why removing the wheel would allow the post to slide out. Does the stem and all the stuff it's attached to just slide out of the whole thing? Or will it be simpler to loosen the packing nut (that big bolt that appears to attach the wheel/handle and stem to the big meaty "T" section of the whole valve assembly) and just put a new wheel/vale assembly in? Or, do I need to remove that whole big heavy "T" part and solder in a new one?
I apologize for my lack of knowledge here; I've been the muscle on a lot of carpentry projects, but have nil experience with plumbing. Any help, advice, etc., would be greatly appreciated. I am a careful, neurotic, and poor new homeowner.
Thanks for any help, and for reading.
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CompleteNewb wrote:

You may be able to stop the drip merely by tightening the packing nut.
Don't attempt to disassemble the valve.
If tightening doesn't get it, remove the handle/wheel and then unscrew the packing nut. Wrap packing material around the stem a few turns and then reinstall the nut and tighten. http://www.azpartsmaster.com/shopazp/detail.asp?PRODUCT_ID083A
Jim
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yeah tighten packing nut a 1/2 turn, quick easy fix.
i have been replacing all my valves with ball valves they last forever and dont corrode
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I also highly recommend replacing it with a ball valve.
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Your first course of action is to tighten the packing nut.
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