The group settings tool gives you access to all the configuration options for your group. It's accessible via the control bar, first click the tools button:
The tools menu gives you editing options as well as the option to add new modules to the page. Clicking the add button will show you a menu of available modules, select Group Settings:
The Group Settings module is shown as a dialog, it will only appear on the page until you log out or change pages. As a group administrator you can give other members access to the Group Settings tool. To do this just add them to the admin/admin permission group and the menu option will be displayed for them too.
You can click through the different sections using the previous and next buttons, you only need to set options that are relevant for your group.
Some options are only relevant when other options are selected, so different sections will be shown based on your choices. Remember you can always come back and update settings later as you become more experienced with the software.