Well, your options are FOWL (Fish only with live rock) or a Reef setup. Personally I would go for a "nano" reef setup.
There is LOTS of information in various threads and stickied topics in this part of the forums so all I can say to do is read. There is no point us going over what you need again and again, all you have to do is browse through some threads yourself, and see what people with similar sized tanks are doing.
Make sure you are aware of what equipment, and what style of ecosystem you are going for as there are many to choose from. Personally on a tank that size I would go for the Berlin method (Live rock, protein skimmer + powerheads).
Your tank is about 18 Gallons.
Ben