Using the Brushes Dialog Box
  When you choose Brushes...  from the Options menu, the
  Brushes dialog box appears.
  To shape the tip of the Paintbrush:
  1.  Click on the brush shape you want.
  2.  Click on OK.
  3.  Move the pointer into the drawing window and try out
      the new brush stroke.
  Additional topics:
  *   Using Menus and Controls
  *   Overview of Paint