You need to make sure you have a female and a male. The female will have splotches of pink on her (the male won't). No pink = no female. They will breed when they are old enough and ready to do so.
Are you sure you have a male and female? If you do you shouldnt have an issue. I have a pair that have bred in a holding bucket and I currently have a pair that are guarding fry in a turtle tank.