I see you posted this in both the C# and C zones.
So far, the replies have been about C#, but the first line of your question suggests that you're using C. So I'll assume this question is about C and not C#.
You can use sprintf to construct a string like this. For example, see the below code sample.
Note that an int does NOT contain leading zero's - it simply contains the integer value. Adding leading zero's is a formatting issue, and you'll have to take care of that when you output the int value somehow. In the code snippet below, it's done using %03d, which means that an int (d) should be shown, that it should take up minimum three positions (3), and that it should be pre-pended with zero's if needed (0). Refer to the reference page for sprintf for more information :
http://cplusplus.com/reference/clibrary/cstdio/sprintf/