I'm glad that you are taking the time to do some research before breeding.
Before breeding your dog you should get the dog health tested for any potential diseases prevalent in the breed. This helps ensure healthy puppies. Different breeds have different health problems associated with them. In many breeds Hip Dysplasia is a problem and is generally tested at 24 months. Chihuahuas have several health issues which you can read about here:
This helps to be sure you are not breeding a dog with a health problem that potentially could be passed down to the puppies.
Male dogs can breed as young as 6-7 months though it is recommended that you wait until they are mature in mind and body which at a minimum I would say is 18-24 months old. The AKC will not recognize a litter sired from a male that is under 7 months of age or over 12 years old unless special documentation is provided.
Female Dogs go into heat at any time between 6-12 months usually. Again, you want to wait until they are mature before having to raise pups of their own. Also breeding a younger female can result in additional problem such as smaller litter size and additional health problems in the mother. I recommend 24 months as well for females. The AKC will not recognize a litter from a female that is under 8 months of age or over 12 years old unless special documentation is provided.
You may find this site helpful if you are considering breeding
Many people breed dogs as small as yours, however, chihuahua frequently need c-sections to deliver their puppies, so I would plan on the expense of an x-ray the last week of pregnancy and the cost of a c-section just in case it is necessary.
Hope this information is helpful to you.