Please start at The Varieties of Data and follow the "Next" links (and the corresponding sections in the DrScheme Companion) until you get to the section entitled Syntax and Semantics.
If you have time, read the additional section Compound Data: Lists of Arbitrary Length, which introduces recursive data structures and recursive functions: they're much easier than they were in Java!
As always, try as many of the exercises in the text as you have time for.