Music theory essentially is the written language of music and the study of how music works. It will not only help you read the written notes of a piece of music, but it will help you identify other important aspects such as rhythm, structure and melody.
The elements of music are the different aspects of music, and each one can be studied in musical theory. These elements are pitch, rhythm, melody, scales and modes, dynamics, harmony, consonance and dissonance, structure and form.
Learning theory mostly requires a competent teacher or book, and some study and practice, like many other skills. Many programs begin by teaching how to read notes, as well as other concepts like scales and intervals.
Music theory allows those trained in it to communicate clearly and concisely when speaking about music. It allows for choirs to perform more elaborate pieces and add more layers to a performance, all with less confusion. Also, the more skilled and trained each member of a choir is, the better the choir is overall.
There are many places where you can learn music theory, both as a solo singer and a choir singer. If your area has a local community chorus, you can start singing with them. You can also take classes, either with a solo voice coach or at a theater/voice school. Many colleges offer basic theory, choral and vocal music classes as well.