how do i find out the DPI of a photo ?

How do you submit the photo? A printout or an image file?

A printed image has dpi - compute it by the number of dots printed per inch on the paper. If you print a digital images that would be the width measured in pixel. When you submit your photo as a file, make sure the pixel size is sufficient for the desired print size of the photo.

how do i find out the DPI of my photo ?

A digital image has no dpi, unless you print it. What is the pixel size of your photo? Select your photo in iPhoto and open the Info ℹ pane.

It will show you the width and height of your photo in pixel below the name of the camera; in the example above the image is 3000 pixels wide. Printed at 300 dpi it will be 10 inches wide. You need to know, how large the calendar is supposed to be printed, to see, if your photo has enough pixels.

How many pixels - width and height - does your photo have?

DPI has no meaning unless there's a specific print size involved. Find out what the calendar size will be in inches (the photo part) and multiply the longest dimension by 300 to get the minimum pixel dimension required for the image. For instance if the photo part of the calendar is 11 x 14 the minimum image size would be 3300 x 4200 pixels. You can submit any image that is that size or larger.

More on this can be found here: The Myth of DPI.