My dog is in heat, I think the time span is 21 days, which days are more risky for her to become breed. I don't want that to happen, does anyone have any tips to prevent this from happening.

Are you asking how to keep her from breeding? Keep her away from male dogs and then get her spayed. Or get her spayed now.

Well, you could try (as Judy said) to keep her away from male dogs. That's a 100%, no-fail plan.

Spay. It's another tried and true method of controlling breeding. I recommend it.

When the dog is in standing heat, which I think is in the middle of their heat cycle, that is when they are more susceptible to being bred. But, I think that the two posts above couldn't state it more clearly... To prevent puppies, keep her away from an intact male. I'm not going to pressure you to spay your girl, but if you do indeed what to breed, then consider everything, from the proper male, to finding the puppies FOREVER homes. I am not one for backyard breeding, but I do realize some people just don't like the idea of surgery on their dogs.

I once had a dog that would do her heat cycle by the book. She was bred once, and ever since then any time a full male was around for a number of days, she would throw herself into standing heat. It was very miserable... and she was a show dog at the time, so spaying was not an option...

They make doggie diapers that can be put on females in heat.

A dogs heat cycle works in a similar way to a woman's, the most fertile time is right in the middle but I have heard of dogs getting pregnant on day one right up to day 21.
Just keep her away from other males and you will have no problem. If you have a male dog in the house this might be the perfect time to get him neutered as he would have to be separated from her anyway while he is healing.

As soon as her heat is over you can get her spayed.

The doggy diapers are a good idea, we used to have ones called B*tches Britches, only problem was we could buy the device but no pet stores sold the refill pads!