Question

how do we apply a filter to an image in photoshop

Answer 1

....Ya bhai its very easy

And you can get this option easily

In starting

Answer 2

↑ Here is your answer ↓
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Shortcut key: Ctrl + F

If you are using Adobe Photoshop CS6, then you can find the filter option in the horizontal menu.

Please refer the attachment below:

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Glad to help you.
@EmadAhamed