Starting from scratch hmmmmm ok.
1. Buy the largest tank you can afford. Larger tanks equal more water and a more stable aquarium.
2. No matter what type of setup you are going for (reef, fish only etc) get a reef ready tank, ie drilled for a sump if you can afford it. It will allow you to hide equipment, add makeup water and suppliments more easily and increase overall water volume (again more water more stability).
3. Don't skimp on equipment. Whether it is a protein skimmer, lighting, heater, sump etc the better equipment you buy now the less problems and added expense through upgrades you will have to deal with later.
4. Decide what you want to keep before you setup the tank. Research those items before you buy anything and plan for their care accordingly. So many people set up a tank with the minimum for fish and then start buying anenomes and corals only to find out they will die without hundreds of dollars of upgrades they have to get yesterday. Your tank will constanly evolve but keep it in the realm for which it was started. In other words don't put an anenome in a SPS (small polyped stony) tank and don't put SPS in a fish only tank. If you want a mixture plan for it from the begining.
5. After all the research and questions and finally the decision on which method or system for your tank you will subscribe to (ie berlin, DSB, wet/dry combo etc) the one thing you will have to have in abundnce and comes very hard for most is PATIENCE. Go SLOW and don't be afraid to post with questions. The hardest thing I did was let my tank sit around empty for months while I was saving for equipment.
6. Regular routine mainence and schedules. After the cycle change the water regularly. Put additives in you tank the same time every day. Feed the same time every day. Have the lights come on and go off the same time every day. These routines will help you to remember to do all you need to and give the inhabitants a more stable environment.
Before this gets to long winded(ok more long winded) I'll wrap it up. Post what you would like to keep and I'm sure the specs for the tank will start pouring in. Then make a decision based on how much time and money you are willing to devote to your new piece of the ocean. Hope this helps
SiF