There is no set rule about when a dog comes into heat. Over the years I have had some come in first time at 7 months and others not come in until 24 or 26 months.
Usually, but not always, larger breeds come in heat later than toy breeds and smaller dogs. My labs normally had their first heats at 14 to 18 months old on average.
Time of year also can have a bearing...many dogs cycle in late Spring and late Fall.
There is absolutely nothing you can do to bring a dog into heat sooner than nature intends. If you are looking to breed her, you need to wait until she is at least two years old anyway so that proper health testing such as hip dysplasia, eye clearances, heart clearances, etc., can be done since most of those certification organizations will not accept tests unless the dog is at least two years old.

Toto, CD, RN, CGC answered on 2/17/12. Helpful? / 2