Unfortunately it is currently not possible to add a bleed value to your Photoshop documents when creating a new one like it is in InDesign or Illustrator.
You need a bleed area in your document (additionally added to your final sheet size) if your design will be printed and colour or objects reach the edges of your document. The bleed will be cut away during the production process. Extending the colour or objects into the bleed area will avoid the appearance of ugly white areas at your document edges. See this animated explanation of how it works.
The action to set up the bleed area in Photoshop contains 2 steps:
Step 1: The canvas size of your original document (1) will be enlarged by 3 mm all around (2).
Step 2: Guides are added with 3 mm of margins all around (3) to show you the bleed area.
Installation:
Download the action and unzip it
Under “Window > Actions” open the Actions panel.
In the flyout menu choose “Load Actions…”
Loacate the “Bleed 3 mm.atn” file and open it
Now you should see the action in your Actions panel
Select the “Bleed 3 mm” action and hit the play button to perform the application of the bleed.And action!