Welcome MonthOldPickle! And might I add, that is one of the most interesting board nicks I've seen in awhile
It sounds as though you're making the switch from Fresh Water to Salt Water? The concept is the same - keeping animals alive in a glass box - but SW will take you a few steps, and a turn or two, beyond most FW aquariums.
If I'm right about the FW, has the 55 you will be setting up previously been used for FW fish? If so, it's very important that you know whether or not that tank has ever had medication used in it. You may have some heavy duty cleaning to do if so.
Since you are on a budget, having a clear idea of what you'd like to keep in your aquarium is the only way to keep costs down and provide the best environment for the animals you choose. Fish Only, (FO) which in most cases is actually Fish Only With Live Rock (FOWLR), or reef set ups which can be a mix of fish, invertebrates (shrimp, crabs, snails) and corals, either soft or stoney. Stoney corals (SPS) and clams are going to cost the most money to keep, in most instances.
If you're handy, there are some great DIY projects to help with your budget.
Books! The old adage was "the first $200 you spend in this hobby should be on books." The internet has replaced some of that need but there is no subsitute for a good book. The above mentioned books are great; my personal favorites for beginners are the Tullock books.
Hold on tight, you're in for a wild ride. Welcome to the addictio...err, I mean hobby!
Alice