Read the tutorials for extending a basic site

http://doc.silverstripe.com/doku.php?id=tutorial:2-extending-a-basic-site

You add something like this to your class, directly taken from the tutorial:

function getCMSFields() {
$fields = parent::getCMSFields();

$fields->addFieldToTab('Root.Content.Main', new CalendarDateField('Date'), 'Content');
$fields->addFieldToTab('Root.Content.Main', new TextField('Author'), 'Content');
   
return $fields;
}

The line in the above code Root.Content.Main refers to your site structure.

Root is the start/top level, Content refers to the content tab, and main is the main tab. Just change the name to a new tab and it will make it for you.