Personally I recommend doing your own thing in ports - especially in the Mediterranean where it is all so easy - but if you want to go with the tours on the ship then you go to royalcaribbean.com > before you board > shore and land excursions. Or you call RCI on 01932 834325 and speak to a human voice.
You can also book a trip at the shores excursion desk on ship. Not impossible you might meet up with someone and want to do the same tour they are doing, but you never thought about it.
And if you join the rolcall you can chat with fellow cruisers before you board and perhaps arrange to do private tours with them?