Where does water that drains off of your street and into your sewers go? More than likely, it goes into a local body of water like a river, lake, or ocean. Protect the water by labeling sewers and storm drains. Stop people from littering and polluting run off water.
Get permission from your local government. Using the city government's website, send an email or make a call. Explain your plan, and offer to share your final design with them.
Come up with a slogan and design. Keep it simple. You want to catch people's attention and allow them to read your message quickly. Think of something like, "Don't litter! Sewer runs into stream!" and add some drawings of fish.
Work with some of your favorite artsy friends to design a stencil (or several) for your slogan and images. Again, keep it simple. You'll have a lot of ground to cover.
Get some outdoor paint, like for porches or house siding. See if you can find brands that are environmentally friendly. Select a few colors and a few brushes.
Divide your friends into teams, and start painting the storm drains your community. Areas with high foot traffic should be your first target (that's where most people drop their wrappers and cigarette butts). In a couple of days, you can probably cover the whole neighborhood.