The male and female's genitals look very similar. The male has a penis and an anus, the female has a urethral cone, which looks like a penis, and an anus. The way to tell the gender is to look at the space between the genitals. If there is a space between the "penis" and the anus, it is a boy, if the genitals are touching, it's a girl.
No offense, I am not trying to be mean, but I am thinking that if you don't yet know how to tell the difference between a male and a female, breeding chinchillas is not a good idea. I highly recommend you take some time to research and learn about chinchillas, genetics, and breeding practices BEFORE you buy chinchillas to breed. It's a lot more complicated than just slapping any two chinchillas together. There are a lot of things to consider, including the time and money it can require. Things don't always go smoothly, there can be complications with any pregnancy, and vet bills add up fast. Plus, breeding and caring for chinchillas requires a lot of time and energy. It cannot be a commitment that is taken lightly. PLEASE consider taking your time and doing some a lot more research before you start breeding chinchillas.