On newer bikes it's printed on the lockring, it's usually 40-50 Nm. As for tools you would need a cassette lock ring tool specific to the maker of your cassette, as most cassettes are either SRAM or Shimano, a SRAM/Shimano lockring tool works. As far as a torque wrench goes, you'd need one like Park's TW-2 that goes from 0-70Nm. However, I wouldn't use the TW-2 to torque stuff below 10-15Nm, it's just not as accurate for small torque values as one that is calibrated for 0-25Nm.

In reality, you really need two torque wrenches, but for starters I'd get the TW-1 or the clicker type that Nashbar makes.