Ideally she should be 3 years old before being bred. For a dog its 2 years. I would definitely wait until, depending what size she is. If she's small you might be alright at two two and a half. If she's large I'd wait until she's three or almost three. You'd be well to get her into condition a few weeks before mating her too, lots of raw meat, some veg, etc.
If you bred her too young she'd have a small litter and sickly pups, and she might not be mature enough to look after them. So its worth it to wait.
Hope I helped,
Good luck mate!