Get Nancy and Colin's book for good background and read the articles on this site, there's a lot of great free info right here! After looking at the list of needs on this site, start going to local fish stores and price out the different equipment at different stores in your area- where I am the cheapest place for hardware is a different store than the best one for fish and info! The sooner you convert to salt, the sooner you'll have a cycled tank, maybe Santa can help out sooner that you'd hoped? There are lots of fun fish and inverts that aren't very expensive (the cheap stuff always seems to be the hardiest). Get into the practice of testing your water quality and performing water changes regularly, just getting used to the differences in routine of salt and fresh. You are making a great choice to take some time and not rush into things!
I have a 55 gal, too, and I really like aculeatus, but there are other good choices for that size tank, too! Research the needs of the different kinds of octos and see what's compatible for you. Right now, octos go for about $50, depending on the type and supplier. Look at some of the on- line suppliers and see what they are charging.