i was wondering the same thing, if i was ready. as i got my puffers SPG up i would start looking into protein skimmers, live rock, live sand, started doing my homework a little more. picked up a cheap protein skimmer for the time being until i do my sump and can get an in sump one. then once i was fully marine i just got some live rock and live sand. its not as much as you would think as long as you dont dive into the corals and stuff. i have gone pretty slow, kept my hang on back filters, kept pretty much everything the same as it was with FW, just added a skimmer and live rock/sand. PM me if you have questions and i will help as much as i can, which wont be too much since i just started marine, but i can try
most things you will have questions on i probably have asked already during my set up.