Welcome to MR..you've already made the first great decision!!
Bris makes a good point cause once you see what garbage a rodi takes out of your tap water, you'll never want to drink the stuff again!
But...human nature dictates that you want to fill that tank up today..so best bet is to go buy some rodi water from your LFS to get started.
Watch the For Sale thread below for a used rodi, or you can pick one up for a good price usually in Ebay. I have one from Air Water Ice & couldn't be happier with it and their customer service (for reordering the filters).