To change your default formatting, go to the Editor tab of Tools > Options... and use the format bar and ruler to adjust the sample text. (The blue A button on the left of the format bar opens the font options.) Click OK to apply the changes and close the Options. Your new defaults will apply to all *new* documents you create (in any project); to change existing documents to match, select them in the binder (Ctrl- or Shift-click to select multiple) and then choose Documents > Convert > Formatting to Default Text Style.
There isn't currently an option within Scrivener to insert a horizontal line. An underlined tab would be the way to do this, setting the tab stop wherever you want to end the line.