I've been scratchbuilding for quite a while and I hopefully can give you some advice on working with styrene. First of all, the aforementioned Evergreen is a great place to get all sorts of sizes and shapes of styrene - they also have "starter" sets of various sized pieces that can be used for your projects. Plastruct also has styrene and ABS plastics available. Depending on how much you intend to scratchbuild you can obtain a pretty sizable assortment of shapes and thicknesses for less than $50, and it'll last for quite a while. As far as tools go, you absolutely must have a supply of X-acto #11 blades...I usually buy them in lots of 100 blades. They are indispensable for working with any plastics. Get a few chisel blades, too. The metal ruler is a good idea, and get one that measures in inches and mm...and one that sits flat on the surface, and not raised. A sharp pencil - like a draftsman's - is good to have: a dull graphite pencil will give incorrect, inaccurate measurements and cause problems later on in assembly and fitting. Of course, obey the law: measure twice, cut once! Get yourself a set of small files and a file card for keeping them clean; you'll need sandpaper - I use a lot of 220 grit, and you'll probably need some finer stuff, too, and some Scotch-brite pads for removing burrs. I like to glue a small piece of 220 sandpaper to a flat block of wood and use it as a sanding base for keeping cut edges clean and perpendicular (if you sand by hand, you'll most likely round the edges). I don't know what kind of glue you use, but my preference is Tenax (hard to find sometimes)...glues easily and dries quickly with a good strong bond. Weld-On #4 works good, too.
This is probably much more info than you were looking for, but when someone mentions an interest in scratchbuilding it certainly piques my own interest. You'll find, I'm sure, that once you've entered the world of scratchbuilding that a whole vista of opportunities awaits you (Jeez, I sound like a travel brochure!!) - you'll be able to create your own one-of-a-kind masterpieces, conversions, and superdetailing. I don't use much PE, but when I do it's usually to use the pieces as templates for a styrene reproduction. Google "KFK" and bring up the Kriegsfischkutter I scratchbuilt a few years ago and see what you can accomplish. Hope this helped and if you have further questions, please contact me.
Doug