go ask alice had a great idea with the old boxers or shorts, as I was going to say to go and buy the dog diapers at your local pet store. There are ones for incontinence and others especially for females in heat.
Keep in mind that the average female will go into heat every 6 months, and it can last for up to three weeks. But, it does differ from dog to dog. All I can say is be prepared.
Please tell me that you have an appointment set up with your vet to have her spayed. If she is a puppy, a puppy can get pregnant in her first heat cycle, which is not very safe for the dog.