If you don't know how old a dog has to be to breed then you shouldn't be breeding them. Also, what kind of an irresponsible breeder would give a novice buyer an unfixed dog or cat? stop supporting bad breeders. FFS just adopt a pet. It's not like cats and dogs are worthless and don't have a personality unless they have a pedigree paper proving their pureness.
Proper breeding of any animal starts with knowledge, so please think carefully before breeding your dog. Caring for puppies is a 24/7 job and proper breeders barely break even, let alone make any kind of profit.
With that said, a female Chihuahua will go into heat sometime during their first year (mine went into heat at 10 months). This is too young to breed the dog. Dogs should not breed until they are at least 2 years old. Chihuahuas are the smallest breed of dog, normally between 3-8 pounds full grown. All small dogs, especially the smaller end of the Chihuahua breed, can have many complications throughout pregnancy and delivery. If you plan to breed your dog, please make sure that you have a respectable and experienced Chihuahua breeder teaching you the ropes.
If you plan to breed your dog simply to make money off of the puppies, please don't. Like I said, proper breeding shows little to no profit at all.
I agree with the others that you should not be breeding this dog, but to protect the dog I will give you the info you need. Because in my experience, people who backyard breed won't listen to reason anyway. Most dogs it's 2 years. Chihuahua's and other micro breeds are more complicated because of their size. They should only be bred to a male of their own size or SMALLER, after they are 3 years old. They they need to be given a break of AT LEAST 1 year to 18 months in between litters. Bitches shouldn't be bred after 6 years of age, EVER.