It is common for excursions to not be available on-line until 3 months or less from your cruise. You can find out by accessing your booking at Celebrity's website and see if they are open yet.
Depending on where you are going on your cruise, I usually recommend first timers book tours with the cruise line. However, many ports are quite safe and easy to get around in and you can often find cheaper tours privately. It depends on your comfort level. A cruise ship will wait for passengers if their ship-sponsored tour is late returning. If you book your own private tours the ship won't know where you are if you are late, and they will leave port without you.
Booking earlier is better, especially for the more popular excursions. Some things do sell out.