How old is your dog? If she is a year or less, this will probably be her first heat. In the beginning, she will have most likely a light discharge/bloodywatery. Then she will bleed for approx three weeks. Her vulva will swell, and sometimes can get quite swollen. Some people day the dog's disposition may change-she may get cranky/whiney/restless.
Do not ever have her outside alone-even if you have fence. Wait at least 4 weeks before she is around any males. If you are going to spay her-best to wait until after her heat. Some dogs have a pretty heavy discharge when they are in heat-others not so much-but there will be swelling.