With acrylics, you can paint on pretty much every surface, as long as it's not oily. You can paint on canvas with no prep work or sealer, but I would not recommend it. The raw canvas absorbs a lot of the paint, making your job very hard. The best option is to buy canvas that has already been base-coated with gesso. You can also buy some gesso and coat any canvas or board that you want to paint on. In this case, follow the directions on the container.
For best results, apply two or three layers; diluting the first coat with water, and lightly sanding between coats.