Hi, welcome to the site!
2 inches of sand, live reef sand is best, the next down would be live sand (it's just loaded with good bacteria), then dry aragonite sand. You don't want to use crushed coral, it's hard on the critters that like to dig, and the CUC (clean up crew of snails and such) have a hard time keeping crushed coral clean.
You need enough live rock to build half way up the back of the tank, make sure it's built on the bare bottom and sand around it, you can't build on shifting sand. A rock slide could kill a critter or break the tank. Many SW critters dig, so make sure it's all very stable. Make some caves, just make sure it's stable. You can use base rock on the bottom and use live rock on top to save some money. You can also use décor with a mix of live rock, that's what I do.
I don't know if you have a sump system or not, so I don't know what type of skimmer you would need. I know it may be a little more expensive to order everything on-line, but you can get a better variety of equipment. I have 4 major fish stores around me, and I always order on-line...including ordering my fish.
Now is the time to plan the type of SW tank...reef...fish only...a certain type of fish, like seahorses. You need to build the tank that your desired outcome should be. Pick your all time must have critter, and build everything around that. You should also set up a quarantine tank at this time, so everything can cycle at the same time.
We are happy to walk you through... but a good "how to" book will help you to know what questions to ask, and explain the basics.