Do not tile directly over plywood. Use 1/4" CBU set in thinset or Ditra.
Look through the old threads in this forum. There are a lot of them addressing this. You will probably find answers to questions you didn't know you had. If this area hasn't been tiled before, or if it has, but has had problems with cracking etc., pay attention to threads on deflection.
Anything written by Jazman or Bud is good advice.
FWIW, 12" x 24" is a tough size to start with. If your floor is not perfectly flat you will end up with lippage (where the edge of one tile sits higher than the next). If you insist on starting with this size, look into a leveling system to keep your edges even. Lash makes one, I think Tuscan makes another. There are one or two others out there.
Look through the old threads in this forum. There are a lot of them addressing this. You will probably find answers to questions you didn't know you had. If this area hasn't been tiled before, or if it has, but has had problems with cracking etc., pay attention to threads on deflection.
Anything written by Jazman or Bud is good advice.
FWIW, 12" x 24" is a tough size to start with. If your floor is not perfectly flat you will end up with lippage (where the edge of one tile sits higher than the next). If you insist on starting with this size, look into a leveling system to keep your edges even. Lash makes one, I think Tuscan makes another. There are one or two others out there.