Pick your pictures
Most calendar templates accommodate one picture per month, so you'll need at least a dozen unique images to produce a basic calendar. If you choose a more flexible template, you can create a collage of photos on some pages, in which case you might want to have several dozen photos ready to go. Copy all of them to a single folder--perhaps on your desktop--so you don't have to scurry around looking for photos as you build your calendar. Collecting them in one spot is also a convenient way to ensure that they look good together.
When I create my annual holiday calendar, I try to find photos from the previous year that reflect the seasons. Snowscapes and frozen lakes decorate the winter months; sunflowers and puppies appear in spring. I use back-to-school photos in the fall, and, of course, the preceding year's holiday pictures in November and December.
In general, it's a good idea to edit your photos first--straightening crooked photos, cropping them for maximum impact, and adjusting colors and sharpness. You can use your favorite photo editor, or you can work with the online editing tools at the Website you select for making the calendar.