Write your message and when you are done, send it.
Kmail will ask for you to type the secret passphrase that you made during the gpg --gen-key step. Type it in, and then Kmail will show you a confirmation window of the signed message. Click OK to send it.
If you test it by sending to yourself, you will notice that Kmail displays the message with a GREEN header, with words like this: Message was signed by Fajar Priyanto (Knowledge is power! http://linux.arinet.org) (Key ID: 0x85EEC6A5). The signature is valid and the key is ultimately trusted.
People that have your public key will have that display also, but those who don't have the key, will have the message displayed in yellow with words like this: Message was signed with unknown key 0xBFE7357F. The validity of the signature cannot be verified.
