Kmail's settings window gives you a number of ways to customize Kmail to your needs.
The settings in the Identity tab are fairly straightforward. The Name and Organization fields should contain the appropriate information if you want this information to be automatically included in the headers of any message that you send. The Email Address and the Reply-to Address field should contain your email address. The Signature File field contains the location of your signature file. You can skip setting the signature file if you do not have one. If you do this, make sure that you also uncheck Automatically Append Signature in the Composer tab.