Resealable plastic bags make excellent piping bags because the frosting or sauce won't come out from anywhere but the point that you want it to. Select a plastic bag based on the amount of frosting or sauce that you need to apply to the top of your dish.  Most packages list the interior volume of the plastic bag. This can make it easy to determine whether a bag is big enough for the recipe. A thicker resealable bag, like a freezer bag, is a good choice if you have a thicker frosting that will require a lot pressure to be applied to the bag. You can use a non-sealable plastic bag if that's all you have available, but you'll need to twist it at the top once you fill it to close it. These bags also tend to explode when you squeeze them, so they're not great for thicker frostings. Once you've added your frosting or sauce, seal your bag by running your fingers on the opposite sides of the seal and pressing it together. If you have a plastic bag with a zipper, all you need to do is slide it to the other end of the bag. Push the frosting, icing, or sauce down towards the corner that you're going to cut. You can push the air out before sealing it if you want. It'll make it easier to dispense the icing or sauce but it will also come out quicker.

Summary:
Get a resealable plastic bag and a pair of scissors. Seal the bag at the top to close it.