Hi Guy.
You will need salt, heaters, powerheads, and plenty of test eqipment.
IT is best to read up as much as possible before even starting. There are a few different approaches to consider, mainly dealing with substrates and types of corals and fish you wish to keep. There are things in the marine world that simply don't mix and match.
Also, how much time do you really want to put into your tank? Some are more time intensive than others.