Welcome. Great first tank!
Word of advice for a beginner, try to source well established liverock. I recommend tampabaysaltwater.com for that. Also if you can find natural seawater instead of mixing your own, this is a great way to jump into the hobby without over complicating things.
If you cant find natural seawater, reef crystals salt is the easiest one to find.
Buy and use a tropic marin high precision floating hydrometer and you'll be mixing like a pro in no time.
You want to use 0ppm RODI water to top off the tank and mix salt water.