If you're already interested in doing environmental research the next step is to define what field of study you're interested in within environmental science.  For example, if you're interested in the effects of air quality or long-term climate then your field of interested can be defined as atmospheric science.  Or, if you're interested in topics such as ocean acidification and algae blooms, then you're field of interest is oceanic science.

While environmental science readily lends itself to interdisciplinary work you'll want to develop your own project first and later tie it into other research.  The idea here is to choose a broad topic within the overall realm on environmental science so you can begin to narrow down your project to a manageable level.