You will have to strip all the old paint off. Then stain will be easily applied.
Depending on the look you want to achieve, you can just sand most of the paint off until you can see the wood grain and then white wash or pickle wash it. But if you want plain stain you will need to remove all the old paint completely.
Donna: you will have to strip the paint first.
Thank you for all your comments
We had enamel painted bookcases we turned into curios. We built a storage bench to go under our big tv and painted it to match. When we moved. We decided we wanted more of a wood look.
After very lightly sanding, I brushed on Poly Stain an rubbed it off to sort of imitate a wood grain. It dries pretty fast and you can darken, or lighten it by how much you rub off.