My home was built in 02 with sink in place. I have a single handle Moen kitchen faucet that I am trying to replace with another unit.Under the bottom of the sink thare is a U shaped plate around the h/c water lines come out. Do I start from the top taking apart the faucet to remove it from the sink, or do I go under the sink? If under the sink,how do I get past that U shaped plate?

You will need to turn off water supplies under sink.

START from the bottom.

Then turn on the faucet to clear any water you can from the faucet. (you will still have some water)

I am not sure without "seeing", but if they are the type of "plates" that look like the support the faucet, then you should be able to slide those off after loosening the nut underneath.

It can be quite difficult to work in a tight place, but it can be done. Are there clips holding them in place, or can you see a nut/washer just underneath them?

Cyberheater gave you excellent step by step instructions. Sounds like your faucet's secured by a single nut under the plate. That can be a bear to get at unless you have a basin wrench,(see image). They can be purchased at most hardware stores and can grab on to the most difficult nuts to reach. Good luck, Tom