By far, the easiest way to let the world know your public key is to publish it on the public keyserver network, a global database of keys.
Some people believe it's a bad idea to use a keyserver, because spammers search them for email addresses. While this is true, the amount of spam you risk to receive by doing this is quite small: you receive roughly ten times more spam when you put your email address on a web page or post a message on the Usenet.
In order to publish your key, click on your key and select Keyserver → Upload public keys. You will be given a choice of keyservers; pool.sks-keyservers.net is the one Enigmail uses by default. Major keyservers periodically synchronize themselves.
Click on Ok and wait for your key to be uploaded. Your key is now published on the Internet for anyone to find.
Once a key is uploaded to a keyserver it will stay there forever.
There is no way to delete a key from a keyserver. You can disable the key by revoking it, but anyone in the world will still be able to view all your user IDs (your name and email addresses), including your PhotoID. If you have privacy concerns about this, don't upload your key in the first place.