The first signs of breast cancer is a detectable lump in your breast, however not all lumps are malignant, i.e. cancerous.
You should learn how to examine your breasts and perform the self-examination every month a few days after your menstruation ended. Here is a link to a website that may give you an idea how to perform your self-exam: http://www.acog.org/publications/patient_education...
If you find a lump you must immediately contact your gynecologist as only a health practitioner can determine whether your lump is malignant. Further tests, like mammography have to be performed.
When you examine your breasts, look for lumps or areas that look like a thickening of a tissue; any changes in the shape or size of your breasts; dimples in the skin; changes in the shape and look of your nipples (they may "sink" or shrink and become irregular); you may observe a dark and bloody discharge from your nipples; rushes on a nipple or on the skin surrounding it; you may detect a lump in your armpit. The tumors are detectable when they are the size of black peppercorn.
Pain in your breast does not usually mean cancer. The hormones fluctuate during menstrual cycle and many women experience pain and swelling of their breasts.
Breast pain is normally not caused by cancer, but there is a breast cancer type - the so called inflammatory breast cancer, that may cause severe pain. If you have any doubt or are uncertain, see you medical care provider.
Remember to see you gynecologist at least once a year for a thorough examination and try to live as healthy life as you possibly can.
In radiant health
I just published an article on breast cancer prevention. You may want to read it: http://hubpages.com/hub/Stay-breast-cancer-free