Creating a Dot Pattern in Photoshop is extremely easy and can be an effective method in duplicating the look of pop art style modern works.
1. Create a new file by going to FILE>NEW change the Image Size to PIXELS and make it 30px by 30px for medium dots (like the ones shown above) or bigger or smaller to adjust those dot sizes to create the effect you're after.
2. Use the Elliptical Marque Tool to create a circle to fill this new square file you've made. Use the paint bucket and the color of your choice to fill the circle. Deselect the circle either by going to APPLE & D or SELECT>DESELECT at the top of your screen.
3. Go to EDIT>DEFINE PATTERN and name your pattern.
4. Go to your main picture that you want to add the dots to and create a new layer.
5. Go to Edit>FILL>PATTERN and select from the list choosing the pattern you just created. Your pattern will fill the page.
6. Change the LAYER to MULTIPLY to see the drawing underneath and use your eraser to eliminate the parts of the pattern you don't want to appear in your drawing.