Heat is more properly known as the estrous cycle. The symptoms are a swelling of the vulva and increased urination. The dog will first come into heat at 6 months of age, and then every 6 months. The cycle lasts for 3 weeks, but in some cases it can be 2 or 4 weeks as well.
While she is in heat, you will need to take special care of her. Firstly, don't let her near a male dog unless you want her to have puppies. Keep her indoors, in a small room if you want, and make sure she only goes for walks with you on a leash. Also, when going for a walk, drive some distance from the house, and then take her out otherwise every dog in the neighborhood will be lined up outside your house.
Consider getting her spayed if you do not want to breed her (which is recommended for all small breed owners unless you are an experienced breeder) and you will not have to worry about any of this.
While she is in heat, you will need to take special care of her. Firstly, don't let her near a male dog unless you want her to have puppies. Keep her indoors, in a small room if you want, and make sure she only goes for walks with you on a leash. Also, when going for a walk, drive some distance from the house, and then take her out otherwise every dog in the neighborhood will be lined up outside your house.
Consider getting her spayed if you do not want to breed her (which is recommended for all small breed owners unless you are an experienced breeder) and you will not have to worry about any of this.