This is a simple tutorial for you, Michelle :).

Create a 250x250 pix file with transparent background.
Now choose a round brush, make sure the brush settings are set to 100% hardness. Choose the color you'd like to use for your dots.

Now make one dot, place it right in the center of the square. Then go to Edit and choose Define Pattern. Name it, for example, Polka.

Create a 5x7 document, make sure the background is transparent.
Create a new layer, and then click on the Create New Fill or Adjustment Layer icon , and choose Pattern. You'll see Pattern Fill options, and all you have to do now is choose the dot you've created earlier, adjust the Scale and click OK when you
like the pattern.

Now rasterize the layer, fill the background layer with the color you want, and that's pretty much it!