Registering to vote as a Green is a commitment to building an institution to fight for the world you want. Party affiliation doesn't begin and end with one election. Register to vote or changing your party registration can be done on-line at vote.gov.
The more people who register Green, the stronger the Green Party is, and the more all parties will take green issues and green voters seriously.
Registering Green does not limit your voting options in general elections.
In states with "open" primaries, your registering Green also does not limit your voting options in the primary election. In states with "closed" primaries, however, if you register Green you will only be able to vote in Green primaries. Check with your Board of Elections.
The Green Party represents most Americans' views on issues like health care, a healthy environment, campaign finance reform, corporate power, genetically modified foods, and others. See The Biggest Media Sin, by Sam Smith of the Progressive Review, and read the Green Party Platform.