Photoshop is a digital image editing software tool, and a part of the Adobe line of products. This software does not have a function for placing margins in a document. However, one can place guides in the document, which can be used as margins. Guides do not automatically serve as a printing boundary the way margins do in other documents, but you can use them as a guide to avoid placing images or objects beyond the boundary of the guide, effectively creating a margin.
Open your document in Photoshop.
Decide the dimensions you want for your "margins."
Make the Photoshop ruler visible for your document by checking "Rulers" in the View drop down menu.
Create your vertical "margins" by placing your cursor over the vertical ruler on the left side of the document. Click in the region of the ruler and drag the first guide line to the point you would like your right margin to be. Look at where the guideline lines up on the horizontal ruler across the top of your document, to ensure it's correct.
Repeat Step 4 to create your second vertical margin line.
Create your horizontal "margins" by placing your cursor over the horizontal ruler at the top of the document. Click in the region of the ruler and drag the first guideline to where you want your bottom margin. Check the dimensions by looking at where the guideline lines up on the vertical ruler across the top of your document.
Repeat Step 6 to create your second horizontal margin.
Remember, the guides you create do not automatically prevent printing outside of their boundaries. In order for the guide lines to act like margins, you must not place items beyond their boundaries.