tropical to marine

how would i go about converting my tropical tank to a marine tank, i
want to keep fish and invertibrates (i think) or would you recomend a
learner's setup to get my hand in with marine environments.

i have currently tank approx 4"x2"x2" on a fairly large cabinet, it is
open topped with a suspended light(which will need replacing as it is
not big enough for tank) heater and external cannister filter.

could any one give me a list of items i will need and also items that
are not essential but help

many thanks

gav