Rand McNally and others publish detailed map books of major cities. Find the creeks, rivers, and tributaries, and trace them through their courses. Look for access points. Access can in the form of bridges, parks, shopping center parking lots, churches, schools and other generally public places. Mark these on the map and go on a reconnaissance mission.
Urban fishing requires certain precautions.
Rule number one is to know the neighborhood. Getting mugged by a street gang or shot by a cantankerous resident can ruin your whole day. Respect private property. State laws regarding what constitutes public and private waterways are not always clear. If a property owner objects to your presence, leave.