Turn it on after you fill it up. No need to wait unless you have sand in suspension throughout the water than maybe wait 10 minutes, but I don't bother.
For the dechlorinator, do what the directions say. Some say to dose for full volume of tank. Which would mean if you have a 100 gallon tank and you did a 50% or a 6% WC you would dose for 100 gallons. For Prime this would be 2 full cap fulls. If it says to dose for the actual WC volume then you would dose for 50 gallons if we used the example above.