The first and most important piece of advice i can give is to research the subject read lots and ask lots of questions.
Secondly plan the system before buying it as my knowledge has got more i have learnt alot and am going to incorparate it into my next tank.
The next is to decide what type of tank/fish you want. Fish only, fishonly with living rock or a full blown reef tank. This is a good place to start as you need to know if the fish youd like to keep are compatible with each other, then if they are compatible with a reef tank.
As for the tank itself, well that depends in part to the fish etc you want to have in the tank, as with all aquariums the bigger the volume the better, it doensnt all have to be the tank itself you could have a container elsewhere that contains the vast majority of water in the system.
The methods of running marine tanks are a lot more natural than tropicals, for a reef tank you can use a mixture of a deep sand bed and living rock, you can also use a sump system these are things you have to think about, It would be best to drill your tank so you can have the sump. With a sump in the system you can hide a lot of the equipment out of site heaters skimmers etc.
That brings me to the protein skimmer by the biggest you can get, this takes all the stuff out the water that harms the fish, and imo is a must in any marine tank.
Finally for now is lighting again this depends on what you want in your tank if you want corals the lighting starts to get expensive.
I probably have scared you off now but if you have anymore questions give us a shout and im sure GL will give you his opinions on the subject aswell
B) 
