Cancer happens when normal cells change so that they grow in an uncontrolled way. This uncontrolled growth causes a tumour to form.
Touching or rubbing any part of your body can't trigger this process.
A woman can't tell if she has breast cancer. If she suspects she has possible symptoms of breast cancer she visits her doctor; if the doctor is concerned, s/he refers her for the appropriate tests.
Some information that might be useful to you, as your Q&A (and this question) suggest you're a teenager:
The chances of breast cancer in a girl under 15 are zero. From 15 - 19 they are one in 1.3 million, which is less than half the risk of getting struck by lightning (one in 500,000).
Fewer than 0.1% of all those diagnosed with breast cancer are under 30. A woman in her 30s who was diagnosed with breast cancer would be considered extraordinarily young, and a woman in her 40s would be unusually young.
80% of those diagnosed with breast cancer are over 50, and the average age at diagnosis is a little over 60.
Breasts are lumpy, bumpy things. Even in women old enough for breast cancer, most breast lumps aren't cancer; 80% of lumps considered suspicious enough for biopsy are found to be benign (non-cancerous).
Advice from cancer organisations, including the American Cancer Society, is that the VERY YOUNGEST a woman should begin breast self-examination is 20, and many cancer professionals say 30.
Before that it's pointless; firstly because there are so many hormonal changes that there is no 'normal' - and self examination is about a woman knowing what's normal for her and reporting any changes. And secondly because the chance of breast cancer is almost nil.
Spread the word.