I've been knitting for years and taught a few friends how to start, so here's my two cents on yarn for newbies tackling scarves. Look for cotton blends; they're smooth, not too slippery, and inexpensive if you shop around discount bins or online sales. They hold up well to frogging - you know, ripping out mistakes without the yarn falling apart. Avoid anything too thin or fancy at first, it complicates things. students always rave about how much easier it is with something basic and budget-friendly like that.
One time I had a beginner try a pricey wool right off the bat and she got so frustrated with the fuzz hiding her stitches, yeah, stick to simple stuff and build from there.