well depending on your experience, we need more information. If you have NO fish experience, then freshwater should be attempted first, just to learn some basics.
If you have some fish experience, then read up as much as possible and always play it safe when you start. If "maybe" or "if" is in your decision then dont do it. The to biggest things NOT to do is to do things quickly and to do things cheaply, because only death and loss of $ result. Next, I wouldnt suggest and medium or difficult species. A fowlr (fish only with live rock) will be the easiest, but if you want to move on to corals after keeping a stable fowlr, then dont fill the tank full of fish, just keep a few and keep the bioload low. Next, the size of tank really doesnt matter but the bigger the tank, the harder it is to screw up, but also the harder it is to get back under control. I wouldnt try a nano tank, since it is a really fine balancing act but it is also cheaper to replace if you do screw up. In the end,, just ask lost of questions, do a lot research, talk to a good LFS owner (one that isnt in the business just to sell stuff, but a loyal hobbiest) and be PATIENT!
And if you dont mind me asking, what is your fish experience?
Goodluck