After converting your varchar2 date to a true date datatype, then convert back to varchar2 with the desired mask:
to_char(to_date('01/02/2012','MM/DD/YYYY'),'WW')
If you want the week number in a number datatype, you can wrap the statement in to_number():
to_number(to_char(to_date('01/02/2012','MM/DD/YYYY'),'WW'))
However, you have several week number options to consider:
WW Week of year (1-53) where week 1 starts on the first day of the year and continues to the seventh day of the year.
W Week of month (1-5) where week 1 starts on the first day of the month and ends on the seventh.
IW Week of year (1-52 or 1-53) based on the ISO standard.