That's a very broad question when it comes to saltwater. You have to ask your self how big, what type (FOWLR, REEF, ...) drilled overflows, to have a sump or to not have a sump. What type of filtration you want to do. Defiantly do your research and planning when you get started. The biggest thing to remember throughout the whole process is patients is rewarded.
First I would start with budget that will dictate a lot on size and what direction to get started in. There are lots of great people here who can point you in the right direction so you can easily and cost effectively upgrade as time and budget allows because it is a very addictive hobby.