Glad you're getting a chance to take it out!
Most recommend that the black tank be at least 3/4 full before dumping so you get a really strong "whoosh" to help clean it out. I usually add fresh water to get it that full first because I dump much more frequently than most. Once dumped, add a few gallons of water to ensure solids disolve. If you have a flush attachment on your rig it makes this much easier.
Many (including me) will often times leave the grey tank valve open while set up so that showers, etc just drain directly to the dump. Not many problems reported from this but after >4 years my black tank sensors still work fine but my grey tank sensors read full all the time so......
When you connect to city water, I'd run water for a while through all faucets... hot and cold to ensure all the air is out of the lines, P traps are full, and the water heater is full. If you turn your water heater on without water in it first.... it can be bad.
Good luck. Have fun. .. and quit freeking out.
It'll all be fine.