Put a "," sign on the number

Put a "," sign on the number

i wanna ask. How do i put "," sign on the number whenever the number
length is more than 3, then add 1 "," sign.
for example: I have a number "100000000", and i want computer display it
as "100.000.000,00", how do i do that?
Here is the image:
In picture above, shown that the SubTotal is "10000", i want computer
display it as "10.000,00" and the Total beside SubTotal is "10000000", i
want computer display it as "10.000.000,00".
My question is: how do i do that?
Thanks