-k fromfield,tofield - usually same field, but you can but together field 2+3 using -k 2,3.
If you give only -k 1 the key is from field 1 to end of line.
This enough, dynamic field length:
sort -t ',' -k 1,1 -k 2,2 ... infile > outfile
if field value is number, you can use -k 1,1n to use numbering sort
also option r is usable for reverse order. Read man sort to get more info.