7.4.1. Share your public key manually

On the Key Management window, select your key pair and click on File → Export Keys to File. Again, you will be asked whether you want to include your secret key in the saved OpenPGP key file: make sure you click on Cancel this time. Save the ASC file, which will have a name like :
Firstname Lastname you@domain.com (0x89ABCDEF) pub.asc.
This is a copy of your public key; notice the “pub” word at the end of the filename.
You can now put the file on your website for people to download it, or carry it around in a USB drive to distribute it to people, or send it via email as an attachment.

Concerning this last option, there is a simple Enigmail shortcut for it: when writing your mail in the Message Composition window, simply choose OpenPGP → Attach my Public key and you will see your public key appear as an attachment of the message you're composing.
Note: if you use Enigmail 0.96.0, this command does not work well due to a bug. Instead, choose OpenPGP → Attach Public key... (which allows you to attach to the message any key of your public keyring), then in the Key Selection window select your own public key and click Ok.