Sigma are good quality. Maybe not quite as good as Canon's higher end stuff, but they're very good.
As said the Canon 50mm f1.8 is a good starting point.
My brother went for the Sigma EX DG 50mm f2.8 Macro, which gives you a 50mm prime and a good macro in one, but it's more pricey than the standard Canon.
The 50mm f1.8 was the first decent lens I got after the kit lens. It teaches you so much about depth of field and framing/composing shots (because you can't zoom).
After that I got a Sigma 28-70mm f2.8 to replace my kit lens, which is great.