I have a stamped driveway, and I seal it every other year.

A concrete/masonary supply will carry concrete sealer - I used Mighty Brite or Diamond Crystal. I'm sure they have something like this. It's in 5 gallon cans. You may have enough for several years.

Power wash sidewalk, let it dry over night. Using a pole and a roller apply the sealant. Use disposable gloves, buy a cheap roller, brush, roller cover and tray - throw it all out when you're done.

Generally the sealant lasts about two years.

Definitely! I have a concrete countertop (DIY!) and used an inexpensive sealer - Quickrete Cure and Seal from Lowes - and the concrete doesn't stain. I've had lots of stuff spilled and left on the countertop that would normally stain concrete- it all wiped off with a damp cloth. The sealer was $18/gallon and I used a thin coat on 55 sq ft of counter, I still have most of the gallon left.

Power wash and scrub first, then seal.

Sealing your sidewalk is a great idea, but you should take the opportunity to make additional repairs and changes to the sidewalk before you seal it. Its better to plan plan to work on a sunny day to help cut down on drying time between tasks.