Three main stages (and costs):
Stage 1: basic setup. Stand, tank, pumps, sand, sump, equipment, salt, etc. Get it all up and running, salinity and temp where they need to be etc.
Stage 2: Live rock. You want 1.5-3 pounds per gallon. Good news is you can use up to 80% dry rock which will become live rock eventually. However, you need to invest in good live rock to introduce a lot of biodiversity. This stage includes the time for the rock to cure and the tank to cycle.
Stage 3: Add lighting, skimmer and start stocking. The skimmer can go on sooner, but will be very unpredictable with all the waste form the rock curing. Stocking can stop or slow down if you need it to (life happens and costs pop up). From here out you want to do 10% weekly water changes and the same size water changes more frequently if any problems show up (algae, strssed anything, etc.)
Always use RO/DI water. If the tank is small enough you can just buy distilled water.
A good water change schedule will prevent most of the problems out there, and likely fix most of the ones that do arise.
Feed high quality foods. I only feed and recommend New Life Spectrum. Have other foods too though, depending on what species you have and if they are wild caught.
What size tank are you thinking?
What fish do you want?
What corals do you want?