If you're not sure what you'd like to observe and report, take a look at some general sites to get a feel for what piques your interest:
Nature's Notebook is a national plant and animal phenology observation program helping scientists, educators, policy makers, and resource managers to understand how plants and animals are responding to climate and other environmental changes.
Citizen Science Central is a nationwide clearinghouse for all kinds of public-participation projects in a variety of topic, ranging from astronomy to zoology. Find a project you'd like to join, or register your research project and solicit citizen scientist volunteers.
Bioblitzes are a fun way to join other NatureWatchers and experts in a day or days of intensive recording of species in a particular area. BioBlitzes are scheduled between spring and autumn and are coordinated by different environmental groups across the country. Search online for the 2013 calendar of BioBlitzes
SciStarter offers "Your Research Mission," with a different citizen science project highlighted daily, or choose a project to assist from a variety of locations or times (beach, home, at night, at school).
Project Noah is a tool to explore and document all types of plants and animals, allowing contributors to upload their photos, georeferenced to a map, to share with and learn from citizen scientists everywhere.
The YardMap asks you to draw maps of your backyard, park, farm, favorite birding location, school, or garden to better understand how to manage landscapes sustainably for urban/suburban birds and other wildlife.