How do I download a PGP key

For you to encrypt your email or read others pgp encrypted mail, the first thing you need is a pgp mail app like igpmail or one of the others available in the App Store. Note that the person you are sending to must also be using a properly setup pgp mail app as well.

Then, first read the documentation for the app you choose - e.g. for igpmail see http://openpgp.org/software/ipgmail/

To actual use pgp encryption, you will need the compatible app, a private key and then the public key of the people sending you encrypted mail.