A small amount of bleach can typically be diluted in water and poured down the sink. For larger amounts, check if there are any disposal instructions printed on the container. If not, call your local hazardous-waste-disposal facility for their recommendations.
Keep in mind that certain cleaning chemicals should never be combined — such as bleach and ammonia — and that holds true when disposing of them, too. Pouring these chemicals down the sink at the same time can create a toxic gas.