..... just go get your self a nice magnesium housing study circuilar saw, hitachi, craftman pro series or Ridge. I find most skill saw purchased over at wall mart target are crappy quality, you really have to get their pro line of tools which I don't think they made too many to have it worth a while, they used to be the top tool maker, not now. But as long as you handle it, it feels study and balanced, not cheap feels. you are good to go.
use any straight edge for your guide, double side tape if you have to. masking tape if you can't filp it over to cut the other side. Cut slow and steady travel speed wise. other wise, other ppl have already advise you on the blades and depth and clamp your work piece and what not safety , common sense.
for the straight edge you have to add or subtract that distances between the saw blade and the edge of your circular saw to set the straight edge. dependent on which way you are cutting.
good luck.