I see two parts to a testing plan. First, what do you want the code to accomplish from the user's point of view? Second, what do you want the individual pieces of the code to do in isolation?

For example, if you're building a program to process information retrieved from a remote server, the first type of test needs to explore the program as a whole -- probably including network connectivity and whatever output your program produces. The second type of test is much smaller in scope. You might test that a subroutine or module that parses the information does so appropriately, returning or creating the right data structures.

If you're still not sure how you plan tests for code you haven't written, ask yourself how you know what code you need to write. Once you know that, ask yourself "What's the first test I could write to exercise that?"