First of all, you have to be running python 2.4 or higher.
Second or all, you need to be sure that the decimal package is actually required. The builtin math operators ( see pow() and ** ) would probably be sufficient for five nines of situations.
If you really want to use the General Decimal Arithmetic Specification, which the decimal module is an implementation of, then I'd start with the tutorial in the manual. If you can't figure out what you're doing after reading that, check back.