If you get into the Master's or Doctorate program, you will very likely automatically be paired with a professor or authority figure who will have you assist him/her in various tasks. Some are interesting, others less so.

Offered the position by chair of department. Was asked what type of class I would like to "TA" for. Asked about any special areas I felt I was specifically skilled in. Asked about prior experience in teaching/ grading papers and exams. Showed the lab I would be working and and given some pointers on how to perform.