I only use circulars now for everything, but learned on straights. Either one is fine to start. So get some needles, a smooth, light colored worsted weight yarn and cast on. Don't worry about it being uneven or loopy because even tension develops with practice.
Look at this page for info, too.
Once you have knit a whole lot and are comfortable you can try a simple pattern. You can also get a sample set of circulars from Knitpicks to try circs in different materials. Then if you want buy the whole set.