Usually details of excursions will be sent a few weeks before you sail and can be booked then. Excursions can also be booked on board, but popular ones may be booked up and queues may be long. If there is a trip you don't want to miss book it before you go.
Find the cruise line web-site that concerns you and you will be able to find the excursions peculiar to your trip. You can book them online or wait until you are onboard and do it there. At least you will know what to expect well before the booklet of excursions is sent to you prior to your cruise. ..... Neil.
On the internet 1 month before you cruise. Or on ship.