Join the Plastic Oceans Facebook page, encourage your friends to join and post your photographs of plastic on beaches, rivers etc.
Organise clean-up events, beaches, parks, the riverside - wherever you see a problem. Hold photography competitions then invite the local news media, they can publish your photographs. See what the main products are that are collected and contact the companies that make them.
For example, if there are many Coca Cola bottles, write to them and tell them that their products are polluting the environment. Organise art projects using the plastic that is collected and use them as talking points to draw attention to the problem we are contributing to with our single-use plastics.