Starting in the reef hobby can be daunting because of the number of interrelated decisions to be made. I would start with the most basic ones. One kind of livestock do you want to keep and where are you going to put the tank? Those two decisions will drive the choice of tank size, along with your budget constraints. Be realistic, it is easy to dream of a 12 foot long 500 gallon tank. It is another to afford it, have a place for it, and maintain it. I think the 'sweet spot' for a first or even a progressing reefer is the 40-150 gallon range and either 3 foot or 4 foot in length. The higher end of that is okay for smaller tangs et al. but not so large as to take over the space or require massive expenditures of time and money on equipment and maintenance. You can always go bigger later on.