In the Warner Books edition with the image of the tree and the bird on the cover, it's on page 112.

The exact page number will depend on what version of the book you have, but it's very easy to find. Look for Part Two, where Chapter 12 begins, and then scoot backward just a tiny bit to the second-to-last paragraph of Chapter 11. That's where you'll find the quote:

"'It’s when you know you’re licked before you begin but you begin anyway and you see it through no matter what. You rarely win, but sometimes you do. Mrs. Dubose won, all ninety-eight pounds of her. According to her views, she died beholden to nothing and nobody. She was the bravest person I ever knew.'"

This is when Atticus is talking to Jem about Mrs. Dubose.

If you need to find a quote fast in your text, try locating a full-text HTML or PDF document online of your book. You can usually find one of these by Googling for "[Title] full text," in this case "To Kill a Mockingbird full text." (However, please be aware that not every resource you find this way abides by copyright laws.)

Then search for a key word or phrase from your quote. If you can't find it, look for grammar or punctuation errors in your search term that you could correct. Here, for example, if you were searching for the phrase "ninety eight" without the hyphen, you wouldn't find your quote until you changed your search term to "ninety-eight." Try shortening your search term, too, and using just the most salient word or phrase that's likely to be rare in the text, like "pounds of her."