I travel a lot but never paid attention to how seats are assigned to
passendgers. Do we get our seat when our boarding pass is issued or
when we order our air ticket at the travel agency?

Although it might vary, I've never encountered a case where the seat
was assigned at the travel agency.

For Business class , you have option to ask for seat number at the
time of puchasing ticket (at travel agent), for economy class you can
ask for seat number when boarding pass is issued (depends upon
availability of seat)

Depends very much on the airline too. On many US & Canadian carriers,
you can call the airline & request a seat; on some others, they're
assigned at the gate. Check this site for the best & worst seats on
major airlines: www.seatguru.com

If you buy your ticket online, you can choose your seat.

Contrary to the previous comments, I routinely fly coach class both
domestic and international and virtually always have my seat assigned
at the time of purchase (either from a travel agent, over the
internet, or by phone.)  Typically the only time when you have to wait
for a boarding pass to get your seat is when the plane is already 80%
full at the time of purchase, as airlines typically save about 20% of
the seats for day of departure assignments.

ty :)