If it's pressure treated you need to wait 3 to 6 months before staining depending on where you live to give it time to dry out.
If not there's no way for the stain to soak in and you will just be sealing in the moisture which will cause a mold problum from then on.
If not there's no way for the stain to soak in and you will just be sealing in the moisture which will cause a mold problum from then on.