This tutorial will show you how to make a pattern in which you can use to fill any selection!

To start off, make a new image with a white bg

Make a new layer and make the shape that you want repeated in your pattern. I just made a little colored circle

Select your image and go to edit> copy

Now, make a new image and make sure that the BG is transparent

Past your image into the new file.

Press Ctrl and A to select the whole image, or just manually select your image.

With your image selected, go to edit> define pattern.

Name your pattern whatever you wish and press OK

Now, take out your pain-bucket tool. This is the tools that you will use to fill your selection with your newly made pattern.

When you select the pain bucket you will notice a new set of options at the top of your screen,

Select the drop-down menu and choose "pattern"

Now select the next drop down menu and scroll down to the bottom of your list of patterns to find your newly made pattern

Select it

Now make a select and fill it with your paint bucket tool!