What kind of sealer or stain is best for a deck?

I had a deck built a couple of years ago, and they said to let it go through the season before sealing it. Don't ask me why. It is just treated wood. Should I just use a water sealer on the deck or a stain? Is there a stain that can be applied and not wiped off (like you would on cabinets or furniture)? I'm a 74 year old woman and will probably have to do it myself so crawling around on my hands and knees is just not an option. I would appreciate any advice you can give as to what to buy and how to apply it.
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