Over at cafepress you can sign up to either a free basic store in which you can add yor own designs and log's to your stores or you can sign up to a premium store for a small fee in which you can add as many designs to the cafepress merchandise range as you want.
Making sure your designs are the correct size and clear is key to making your images stand out also you can use the usual promotion methods that you would use to promote a normal business, like newsletter subscriptions to capture subcribers and free and paid advertising.
There are many opportunities for someone who is not very good at designing or art, there are many sites on the internet that have free to use clip art that you can use royalty free for such things like this or anything similar.
http://easydesignknowhow.com/cafepress.php
This is an art and design blog with featured resources and other websites that have information on a wide variety of art topics and subjects of relevant interest.
