When Should I send mail to a university professor for ta, ra? i.e. after getting successful admission for MS or after applying to the university? If I send mail to the professor for ta, ra after applying to that university is there any chances to get successful admission confirmation to that university?
This depends on the university and the field. In some fields/places the professor has little say in who is a TA or RA. In others, the professor actually hires the RA directly, especially for grant-funded positions.
Likewise, in some places/fields the professor has little say in the admissions process. In others, he/she has a large influence on who is accepted.
So, depending on your case:
If the professor is directly involved in admissions, contact him/her early.
If the professor isn't involved in admissions, but does hire TAs and Has directly, contact should come after you get admission. You can write earlier to express interest, of course, but not to request (or expect) and action.
If the professor isn't involved in either admissions or funding, you can probably wait until you actually arrive, though expressions of interest can come earlier.
If you send too much to a prof too early in the process it will probably be ignored since they may have no influence and so evaluating it is just wasted time. After you are admitted, it becomes more meaningful to them as well as to yourself.
How and when you get confirmation depends on all of the above as well as the rules of the university. And different fields have different traditions.