There are two main national psychology organizations in America: The American Psychological Association and the Association for Psychological Science. Both organizations include resources for students and offer student membership at a discounted rate.
The South Western Psychological Association is a regional psychology organization. Its annual conference is a favorite venue for UTPA psychology students. Membership and conference registration is well below that of the national conferences.
"The American Psychological Association is the largest scientific and professional organization representing psychology in the United States. APA is the world's largest association of psychologists, with more than 137,000 researchers, educators, clinicians, consultants and students as its members. Our mission is to advance the creation, communication and application of psychological knowledge to benefit society and improve people's lives."
"SWPA works to promote and strengthen psychology's scientific, professional and educational facets. At its annual meeting, SWPA offers paper, poster, continuing education sessions, workshops, symposia, and invited talks. SWPA prides itself on serving the psychological community by providing access to scientific advances and professional development within a collegial atmosphere. Students are welcomed and treated as active and valued participants in the discipline."
There are also many specialized psychology organizations that focus on specific topics in psychology. For a listing of some of these organizations, along with listings of psychology organizations based outside of the USA, visit this site.