I want to make snickerdoodles but I don't have cream of tarter, corn starch, or shortening?

If it calls for baking soda and cream of tartar, you can replace the two of them with baking powder. (That's what baking powder is.) You can replace shortening with er or margarine. I don't know about the corn starch - I've never used that in any cookie I've made. I'd say, you could leave that out, and add more flour if necessary.