This is the code I used to get that ball bouncing around my screen. See if you can go through it and understand it. Then try making some changes.

import sys, pygame
pygame.init()
size = width, height = 320, 240
speed = [1, 1]
black = 0, 0, 0
screen = pygame.display.set_mode(size)
ball = pygame.image.load("ball.png")
ballrect = ball.get_rect()
while 1:
  for event in pygame.event.get():
    if event.type == pygame.QUIT:
      pygame.quit()
      sys.exit()
  ballrect = ballrect.move(speed)
  pygame.time.delay(10)
  if ballrect.left < 0 or ballrect.right > width:
    speed[0] = -speed[0]
  if ballrect.top < 0 or ballrect.bottom > height:
    speed[1] = -speed[1]
  screen.fill(black)
  screen.blit(ball, ballrect)
  pygame.display.flip()