If statement help on when to use commas and spaces

Basically I've built up a string and I need to put an if statement on when to use a comma and a space. So basically I need it be after the first element and not on the last element.

s += firstPart;
s += secondPart;

So when I test it out it does:

firstPartsecondPart

and I want it to do

firstPart, secondPart

Thanks in advance.