Open Photoshop witht he document you want to put a brush on.
Select the Brush or the Pen tool which looks like that:
or
I prefer using the Brush tool. If you don’t see the brush tool but only the pen tool, right click on the pen tool and you can select the brush tool
Now that you select the brush tool, you need to select the brush you want to use. There are a few ways to see the list of the brushes, but I will show you my favorite one.
You should see a bar at the right, with a little paint tool, well select this and the list will open like you can see in the picture below.

If you don’t see this, you can do it another way, click look at the top of your screen and click on the little arrow beside the brush sign which looks like that:

Then select the brush you want to use by clicking on it and you’re done now you can stamp the brush on your picture.