to expand on what he's saying, you want two velocity values for simplicity.

x = x + vx

y = y + vy

when the ball hits a paddle or the top/bottom, you just do the like:

if y > paddle1 and y < paddle1 + 20 and x < 0 then vx = -vx

if y > paddle2 and y < paddle2 + 20 and x > 319 then vx = -vx

if y < 0 or y > 199 vy = -vy

the top two lines reverse the x speed of the ball if it hits a paddle at location paddle1 on the left and paddle2 on the right. the second changes the y speed of the ball if it hits the roof.