Research, planning, and time are key. You should have a complete plan for everything down to every pipe, tube, and wire before you spend a penny on equipment, the same goes for livestock.
Unless you get a really good deal on a used setup know that you are going to be spending at least 1000.00 for an entry level 40-75 gallon setup. If you want the latest tech, then the sky is the limit on what you will spend, as there is always bigger, better out there.
Know that a good looking successful tank requires consistent maintenance. You can not ignore a salt water tank the way you can a fresh water tank. If you do you will pay for it, as it will show it's displeasure 20x as fast as a fresh water tank.
Ask questions of this great online community, we are all happy to provide our advise. I wish I had before I started my tank, as I would be much further along than I currently am.
It is never a good idea to buy less than what you believe you need for anything, replacing lesser equipment for what you should have gotten in the beginning gets very expensive. If you do not have enough to get what you really want / need then wait and save up more money.
Constantly question everything and anything at a LFS (in your head at least), few get much more than 50% correct, and I have left the LFS with out what the livestock I wanted just because something did not seem correct.
QT all livestock. This can not be said well enough, if you setup a QT from day 1 you will never regret it. If you do not and go with out for a bit one day you will, and on that day your entire tank and everything living in it will be at risk.