Hi, jessiaeh16. My name is Steve. We will need to have more information for a more exact answer. Some teachers can spend weeks on this topic alone. In short, to add or subtract matrices, they have to be the same sizes. If they aren't, you can't add/subtract them. To multiply them, their sizes have to be "switched". For instance, you can't multiply a 3 by 2 matrix with a 3 by 2 matrix. But, you can multiply a 3 by 2 matrix with a 2 by 3 matrix.
Don't hesitate to post some more. We are always willing to assist.