Once You know what size of tank you want,how much money you want to invest,how much upkeep time you want to spend,and finally what kind of creatures you want to house? Fish only tends to be the easiest and full reef tanks being the hardest,generally speaking.There are many threads about tank setups and similar topics but like many things there are many ways to do things the right way. I can recomend a couple of good books such as Marine Reef Aquarium Handbook and nano reef aquariums.The later is good even if you are aiming for a larger aquarium as it is rather comprehensive.