There will be certain speciality restaurants and entertainment you can book before you get on the ship, notably the Limelight. This normally opens up 2 weeks before you cruise via the P&O personaliser webpage. Well worth being on at midnight to ensure you get what you want. Everything else will be first come first served on embarkation via the on-ship webpage (MyP&O), so make sure you are looking at this tootsuit after boarding. It is normally cheaper to pre-book restaurants too, so worth doing.