[The material on Trees under Howson is probably more current than this. These notes will be updated, though (Nov 2011)]
This section of the tutorials and
A.Hausman, H.Kahane, P.Tidman [2007], Logic and Philosophy
would work well together.
You need to know some sentential logic to be able to understand the tutorials to come. In particular, you need to know about the symbols used in sentential logic, truth tables, satisfiability, consistency, and semantic invalidity (by counter example). You do not need to know sentential rules of inference and derivations.
Hausman [2007] will give you enough background.
Alternatively you could look at the first four sentential tutorials in Easy Deriver: Hausman
You should perhaps also check Checking your computer is configured correctly and Writing Symbols (and Alternative Symbols)
Most of these pages load java applets and that means that they may take 20 seconds or so to load. [Your browser will then 'cache' the applets and subsequent work will proceed much more quickly. Recent Apple system software is not our friend here. OSX Tiger loads these pages in 10 seconds on old Mac FlowerPot. Whereas OSX Snow Leopard loads the same pages, containing the same software, in about 1 minute on a fast and recent MacBookPro.]