My advice is get as big a tank as you can afford. Also make sure you have a lot of money and time to invest. This is a VERY expensive hobby. For example, a 55 gallon reef tank is going to cost well over $2,000.....maybe closer to $3,000 once you get all the animals in.
Make sure you have a place away from windows to place the tank. Also remember that a full 55g tank can weigh as much as 800 pounds! You might have to research your floor plans if you live in an old house or an apartment.
Also make sure you have an area you can put the tank that doesnt have furniture nearby that is easily damaged by saltwater. Saltwater is very corrosive to all sorts of metals, wood, leather, etc..
Research the lighting requirements for the types of animals you wish to keep. This is where the vast majority of beginners make a mistake and the result is heartbreaking.
My last tip is my most important one in my opinion. Make sure you shop at ALL the local fish stores, compare prices, and HAGGLE.
Welcome to the hobby! With patience and disposable income, this hobby is one of the most pleasant and rewarding experiences you can have.