There is alctually no set standard, the values of m are just "labels" for the posibilities, it is actually up to the textbook that you are using or your teacher/professor.
Nonetheless, most college textbooks of general chemistry, including
General Chemistry by Chang
Chemistry by Silberberg
Chemistry by Freeman
Chemistry by Atkins
will typically asign the following
px = -1
py = 0
py = +1
dxy = -2
dxz = -1
dyz = 0
dx2-y2 = 1
dz2 = 2
The p assignments are the most common
The d assignments actually vary more even from one authors edition to the next.
Thus remember that the assignment is arbitrary, however, I always tell my students that for purposes of my class they should use the above assignments.
Valdo