Here's a link to
garf look under diy pages for the tank info. Plywood , was mentioned, glass and acrylic as well. Acrylic would be IMO the hardest to build if you have not worked with acrylic before.
Glass, you can have the glass shop cut it for you, just remember that the sides sit on the bottom (all sides).
Acylic- less $$ to get a 1/2 sheet, 3/4 sheet or a full wheet depening on the size of the tank. For a 55 popbably a 1/2+ will be enough for you.
Plywood-the eaisest to build = using ac/x ply and glass. You need to use a wateproof glue, screws. Just "butt" joints work.
hth