7/26/08 10 Software
R.C.Jeffrey,  Formal Logic: Its Scope and Limits Chapter
Jeffrey  has a number of exercises. Many of them you will be able to do in the Applet below.
Here are a few hints
A Simple Tree
[This is a Quicktime Movie, click the Play button to view it. The logical symbols you see in use may be different to the ones you are familiar with (sorry about that, but it is not practical to produce different movies for all the minor variations in symbols). Any differences will not affect the principles being explained here.]
This section of the tutorials and
Richard Jeffrey, [1967-2006] Formal Logic: its Scope and Limits
would work well together (and it is an excellent book).
You need to know some propositional logic to be able to understand the tutorials to come. In particular, you need to know about the symbols used in propositional logic, truth tables, satisfiability, consistency, and semantic invalidity (by counter example). You do not need to know propositional rules of inference and derivations.