AdiIRC has a lot of options which can be confusing, however the defaults are good enough in most cases.
You can narrow down the option you want by typing in the Searchbox at the top, it will mark the matches with a red rectangle.
Example search for "Spacing":
You can reset all options in all tabs by clicking the "Reset" button.
If you want to reset only one tab, you can right-click it in the listbox and chose "Reset".
Clicking the help icon, opens the help page for current tab with more information about each option.