Before you take on turtle or tortoise care, it's best to research both animals since there are very distinct differences between these two types of reptiles. Reptiles Magazine is probably one of the best places to start learning about turtles and tortoises. As a rule, tortoises are land-dwelling, while turtles are marine.
One of the main differences between tortoises and turtles is that most turtle species are chiefly carnivorous, while most tortoise species are basically vegetarian. However, there are always some exceptions so it is prudent to check the exact requirements of the species of reptile you intend to care for.
"Tortoise chow" is available, but it is much more expensive and frankly not as nutritious as the fresh vegetables such as romaine, red leaf, green leaf, collard greens, and turnip greens most tortoises truly enjoy. Our tortoises also enjoy fresh dandelion greens and plantain along with the occasional treat of radicchio.
Most turtles are omnivorous. In the wild they eat insects, fish, worms, snails, other marine animals and water plants. While you may feed them gut loaded insects, such as superworms, and green plants such as romaine and green leaf lettuce from time to time, most turtle keepers find it easier and more economical to feed pellets that are made especially for turtles.
Both turtles and tortoises need a fair amount of calcium in their diets. Their shells are actually an external skeleton. Not enough calcium will result in poor shell growth and poor health.
Most reptile traits are what you can call “hardwired”. They are built in. Reptiles don’t operate on emotion. Their code is eat, evacuate (poop, etc.) , mate, sleep. Whether you keep tortoises or turtles, they will likely never show affection for you and in fact will probably never recognize you as a caregiver in the same way as a dog, cat, or other companion pet would.
Some species of turtles are protected in their native areas. In other words, it is illegal to keep them in captivity without special permits which range from difficult to impossible to get. For instance, where I live, Blandings, Painted Turtles, and Map Turtles are native. While we can keep Painted and Map Turtles, Blandings are protected and are not allowed to be kept captive.