I'm assuming you are thinking of breeding her. Let me explain how the heat cycle works for you which will aid in any breeding plans you have.
A few key things is that the female needs to be in the right stage of heat and also needs to be brought into the males territory for mating and not the other way around. To breed successfully you need to understand the heat cycle. I have explained it below and included a site that has a pictorial guide so there is no mistake what stage of heat your girl is in.
Female Dogs go into heat at any time between 6-12 months usually. The average heat cycle for a dog is approximately 3 weeks but for some dogs it is longer and for others it is shorter. Some as short as 10 days and some as long as 4 weeks or more. Usually your dog will go into heat every 6-7 months though again each dog is different and can go back into heat as early as 4 months or not for 12 months. Most however are average.
During the first part of a dogs heat period you will notice swelling of the vulva, possibly an increase in urination and bleeding. If there are male dogs around you will notice them hanging around her. This stage generally last about 7 days and she will not allow males to mate with her during this period The second part of the heat cycle is when she will accept a male and breed. Usually bleeding has stopped or become straw colored at this point. This can last from 4 to 21 days though the average is 7 days. Many expert say the 9-10th day of heat is the optimum day for breeding. It is during this period of time that you will want to allow mating. Most breeders allow the dogs to mate every other day during this period.
Then during the last stage of heat she will be less willing to breed as she is going out of heat. This stage can again last from 4 to 21 days as well but averages 7 days.
Each dog is different so some evaluation by the owner is necessary. The whole heat period should be approximately 21 days but as stated can be longer or shorter depending on the dog.
Here is an excellent website with pictures to illustrate a female in different stages of heat.
I hope you find this information helpful.