Cotton seems to be the fabric of choice for making cloth masks, but a single layer of cotton on its own doesn’t fare very well in preventing particles from getting through so be sure to use multiple layers of it. In general, the more easily you can see through a fabric when holding it up to a light, the more layers of that particular fabric you need to create an effective mask.
Keep in mind the main purpose of wearing a mask is to catch outgoing particles from infected people before they reach the air, not to stop viral particles from getting in. Most fabric masks do not fit snugly enough to offer excellent filtration of inbound viral particles, which can be smaller than one micrometer in size.
Because the primary purpose of a face mask is to catch outgoing particles, you do not necessarily need to include a filter in your mask. If you do choose to add a filter, considering buying a filter made for face masks, instead of using something like furnace filter material that could contain microscopic particles that would be dangerous to inhale.
Mainly, you should make a face mask made with a fabric that feels comfortable and reasonably breathable to you so that you will wear it regularly. You may need to experiment to find just the right fabric for this purpose.
