For a 55 gal
tank you can use 2X4's to frame the stand and then use some nice quality plywood or paneling to cover up the sides and front. Be sure to put a door in the front and leave the back open.
Think about the viewing level of the tank. Do you want it to be eye height? Do you want to have easy access to the tank with out having to use a step stool.
Its extremly important the stand be level and square. Is this going to be a freshwater or saltwater tank? This will affect the final weight of the tank but either way you can estimate the weight is going to be at least 600lbs. Now I doubt you need to put extra supports on your house or anything. In fact thats equivlent of 3 semilarge guys standing side by side.
To spread the weight out over the most area have the bottom of the stand be made up of 2X4's in a rectangle the same dimentions of the tank. Build another rectangle for the top frame and then build 4 or 6 legs. Screw the legs to the bottom and top frames. Then cover the stand. Paint and stain to what ever color you like.
Those are some simple ideas and concepts.