Breeding questions don't come up here often because many of us feel strongly against breeding our RES...especially due to the gross overpopulation of hatchlings in the pet trade that are not properly cared for and quickly die. Also, some locations have ordinances against the breeding of turtles, because of population/salmonella concerns and the "4 Inch Rule" for commercial sale. Please do some research into local laws, and just like with other animals, make sure you can either find them all quality homes, or can handle housing them yourself, long before you encourage their breeding.
Responses about sexual maturity can vary...some say they need to be at least 3-4 years old, others go by specific size, approx. 5". I believe females mature quicker than males, which makes sense considering females tend to grow quicker and larger. The female, once gravid, will need access to an appropriate soil area for her to deposit her clutch of eggs. If they are deposited in water, most likely they will not be salvagable. Eggs will then need to be kept in an incubator. (Many good herp stores carry decent ones at reasonable prices.)Some breeding pairs will be very cranky with each other if they don't have adequate space.