Is the key that it can go in the oven? Can I use a pot then that is oven-safe? Or do I have to buy something special? Sorry this is such an amateur question, but I need help!
This is a very reasonable question! Casseroles are usually dishes that will go into the oven at some point so, yes, any dish you use for a casserole should be oven-safe. Past that, there is a lot of flexibility in what sort of pan to use, depending on the recipe. Most recipes, like this Tuna Noodle Casserole at Epicurious, tell you what volume pan to use - in this case, a 2-quart dish.
Some recipes, however, will specify the actual size. A very common pan size for casseroles is a 9x13 inch pan. This is the classic casserole dish, and glass or metal 9x13 pan will see you through almost any sort of casserole.
We like this Pyrex 9x13 Dish because it comes with a snap-on plastic lid - handy for storage and potlucks.
You can use other pans too, of course, but sometimes you will need to adjust recipes. If, for instance, you are making a layered casserole that calls for a 9x13 pan, but you put it into a round dish with less surface area, the casserole will be deeper and thicker and will probably need more cooking time. If you put it into a large jelly roll pan you'd need less baking time because it would be very spread out and thin.
Now - back to the readers. Any suggestions for Mary on good casserole dishes?