If you don't have a supervisor, you cannot get RA. Because RA is
given by the supervisor. For TA, if you are Ph.D. candidate, you
have a better chance. As far as I know, no master student
admitted last year got TA. TA is very difficult to get these
years, and I don't know the criteria. I doubt if there is any
critera. Students of some professor may get TA easily. Other
student may never get TA no metter how good they are. If you
got TA, you really have very good luck! And if you got it,
you can hold it for at most 4 terms.