Clear up the red tape. Fill out necessary applications for club status on campus. Read up on all applicable requirements and meet them. Designate a "rented" campus room at a certain time of day and week that works for you.
Come up with a snazzy name. The success of a club depends partly on its appeal to random students. Many college enviro-clubs use generic labels like "Earth Group," you can make yours special by modifying to names like "Earth Focus" or "Planet Fix."
Place flyers around the campus for a "general assembly" meeting. At this meeting, you can talk about aims for the group, get a list of names for a mailing list, and find others who will be involved in the future.
Select officers. From the general assembly meeting, you should have a list of names. Advertise for officers (treasurer, president, vice-president, etc.) on your mailing list. As founder, you can elect yourself president, or enjoy a more consultant-type role like "Premier." Your officers will be your (and the group's) front line people who take care of all practical issues.
Attend meetings regularly to make sure the group is operating well. Before long, you'll be able to sit back and watch your planetary advocacy club blossom and spread out across the campus. Your initial efforts, like a seed, have borne fruit that, with any luck, will affect the campus positively for years to come.