Whom should I report spam emails to?

There are national agencies handling scam, fraud & phishing reports (e.g. USA, UK).

However, this inheritance scam is an example of advance-fee scams including 419 scam, Nigerian prince scam and many other. The email you got is following a so well known pattern that reporting it wouldn't really increase any information on the subject. Scams like this have been around long before email, e.g. The Spanish Prisoner from 1910. From this perspective it might be better just delete it.

It might be useful to report the origin of the spam, though, as it could be a compromised system. Therefore, the owner of the abused system and the network operator should know about the situation. The correct party to contact can be found by investigating the Received headers of the message and looking for abuse contact information in the WHOIS database. As not everyone can interpret this information correctly, there are services for this, too, like the SpamCop.