We assume that the product is already installed. If it's not the case, check the installation instructions.
Default configuration of WebMail Lite assumes that IMAP server is installed on the same machine. This scenario would cover most cases of using the product. When the user logs in for the first time using valid credentials for their account on the IMAP server, the account will be automatically created (auto-provisioned) in WebMail Lite. This new account will inherit all the default settings for language, timezone etc.
If you intend to use WebMail Lite with mail server installed on a different machine, you'll need to configure default domain settings.
By default, WebMail Lite uses email address as mail server login (email@example.com). If your mail server requires just an account name (jdoe) for that, you can alter this configuration option in settings.xml file; it has many other configuration options available.
If you experience any issues with your product installation, be sure to enable logging as the log files can shed some light on the reasons of those issues. Clear the log file(s) and reproduce the problem to make sure it is reflected in the logs.