It has several sections, selectable by the tabs near the top of the dialog box. The first sheet contains the most important options - Full Name, E-mail Address, Nickname and Alternative (Nickname). Full name pertains to what name should be displayed when people get information on you. E-mail address is for your e-mail address. The next two are important, as the Nickname is how you are identified on IRC. Thousands of people use IRC and you cannot use the same Nickname as someone already on IRC. Also note that some IRC networks impose a length limit on your Nickname. Undernet, for example, imposes a 12-character limit. Finally, some characters are defined as illegal, meaning you cannot use them. Your Alternative Nickname is what mIRC should default to if someone is using your primary Nickname. Diagram 2 shows an example filled out.
The next thing you have to fill out is the Identd sheet. Diagram 3 shows this sheet.
This is related to when you connect to the IRC server. Most people put in their Nickname in here. Also check the Enable Identd server checkbox. Diagram 4 is an example of a filled out Identd sheet.
As is, you've filled in the minimum information required to connect to an IRC server.