There are some changes needed, hopefully you can sort through this working example and see what I've done:
def smallest(numbers):
newlist = numbers.split(',') #splits the incoming comma-separated array
newlist.sort() #sorts alphabetically/numerical
return newlist[0] #returns the first value in the list, now the lowest
def main():
input_file = open("num.txt", "r")
number_file = input_file.readlines()
for i in range(len(number_file)):
print smallest(number_file[i]).rstrip() #rstrip strips \n from output
main()