This isn't some Blue Peter project with sticky back tape involved this is a professionally designed and sellable product. There is a company called Lulu.com who help people self-publish books, they also help people design and make calendar and have all the software you need to do this on line. You don't even have to buy them to resell as they are made to order from customers direct from their website. All you have to do is design your calendar and tell everyone were it can be bought.
The construction of the calendar is the easy part, the harder part is getting people to buy them, so you will have to spend a bit of time telling everyone about it, advertise on your own website if you have one, use it in you email signature, ads in local press, shop windows and all the normal places where you will find potential customers.
There is the option to buy some in bulk and resell them for a profit, that way you will have the product to hand to show people, this will be easier to sell, i.e. eBay, car boot sales, jumble sales etc. You will of course be investing and most who want to earn a little extra cash don't want to commit money up front as there are no guarantees.
The calendar will only be as good as the photographs, 12 obviously. Some sort of theme that will appeal to certain groups, i.e., pictures of dogs or cats, dog and cat lovers will buy these or get one as a present for family or friends. You don't have to just have one calendar, but you can design as many as you want to cater for more people with numerous themes.
You don't even have to have the calendar to start from January, you may start from any month of the year, that way sales won't just be seasonal.
Finally, you will probably also save money by using the calendar as presents or gifts to friend and family, not only that it could be made up of family or friends photographs, which is a nice touch.
The site can be found here: www.lulu.com