There are two ways to visit the Mekong Delta. One is on an organized tour. The other is on your own.
To Book a tour around the Mekong Delta before departing to Vietnam, you can go to your local travel agency, or search the internet for Mekong Delta tours. Vietnamese travel agencies will offer a better price. If you are not confident with internet deals, you can wait till you get to Saigon. Any travel agency in the center of Saigon, will organize a tour for you. More or less all agencies offer the same tours, but you should compare prices and services before reserving. If you book with a Vietnamese agency, you will need to be flexible. Do not get upset if there is a change of plans, changes happen all the time in Vietnam. If you book a hotel, and are brought to another, ask for a money refund to your guide.