Welcome to AA! What the others posted about using tap water is true. Using tap water will lead to unsightly algae blooms and basically make your tank look like crap. The fish will survive in it, but your ability to have any sort of coral is little to none. You can find RO
water at your local Walmart or grocery store. What you'll be looking for is the machine people use to fill the big blue water cooler jugs.
It sounds like you have definitely done some research, and I know it has resulted in more questions then answers lol. To break it down into phases you basically have:
Phase 1: This is where you pick out the tank, get your hardware together like powerheads, skimmers, liquid test kits, refractometer, heaters, T5HO lights (if your planning for corals down the road) etc.
Phase 2: This is where you setup the tank, fill it with RO
water and mix in the salt using the refractometer to get to your ideal salinity. After the empty tank has been up and running for around 24-48 hours to let the salt mix and clear, then you can go get around 20-30 lbs. of live rock and put it in. You then can add your substrate of aragonite sand (rinse it well before hand in RO
Phase 3: This phase takes anywhere from 2 weeks to a month usually. This is when you begin the cycle of your tank. There are many articles on how to do this, but just know this has to be done BEFORE you get any fish.
Phase 4: After your cycle is finished, you'll do a water change of around 20-40%. Now you are ready for your first fish! Make sure you get the least aggressive fish first. This will help prevent territory disputes.
That's basically the set of steps you need to get it up and running. After it matures for a few months and you get a good handle on doing weekly water changes and keeping the parameters stable, you'll be ready to shop for corals. You can add fish at the rate of one every 4-6 weeks to be on the safe side since it's a young tank. Too many at once and your tank will crash. A great place to buy from is liveaquaria.com. I have purchased quite a few things from there and they have always been good.
Good luck with you new tank!