@Zeravala's answer is good, but I'd add a few things.
When I was learning Spanish, I decided that the best translation of sino was most often "but rather." No leí el libro de Bioy Casares sino el de Borges translates to "I didn't read Bioy Casares' book but rather Borges'." Of course sometimes "but" works better, sometimes "rather" or "instead", and sometimes something quite different works best, which brings me to my next point.
Sino can mean something along the lines of "do anything but" such as in no puedo sino reír which you might translate as "I can't do anything but laugh." This second usage is far less common than the first, but as it will turn up from time to time, you should be prepared to recognize it.
Finally, to reiterate what @MikMik said, you should also be prepared to see what would properly be written as si no squashed together. Many people are sloppy.