AdiIRC has a lot of options which can be confusing, however the defaults are good enough in most cases.
It can be opened by pressing the ALT + O hotkey, by typing /options or by opening File -> Options in the Menubar.
You can narrow down the option you want by typing in the Searchbox at the top left corner, it will mark the matches with a red rectangle.
For performance reasons, only text/labels and group labels are searchable, dropdown/list items are not searched.
Example search for "Spacing":
You can save options by pressing the "Save" button or press the CTRL + S hotkey.
AdiIRC will also warn you when closing the options window if there are any unsaved changes.
You can reset all options in all pages by clicking the "Reset all" button.
If you want to reset only one page, you can right-click the page in the listbox and chose the "Reset this page" menu item or click the "Reset" button.
Clicking the help icon, opens the help page for current selected tab with more information about each option.
Help for a page can also be opened by right-clicking it in the listbox and chose the "Help" menu item.