The simple rule is, if it says "send this to everyone you know" just don't. Mostly it's virus hoaxes, but occasionally you get an email petition that is quite compelling (but always pointless).
Check
www.vmyths.com for the lowdown on virus myths, and
www.snopes.com for urban legends (including various petitions and chain letters).
In time you'll be able to spot a bogus mail before you've even finished the first sentence.