See the two values main takes? They contain your commandline arguments. argv is an array of strings containing the arguments, and argc tells you how many strings it has in it. In nearly all cases, the first argument is the name the program was run with, and all the ones after it are the ones you want.
Were you to run your program with my_c_program a b c d, then the contents of argv[] would be { "my_c_program", "a", "b", "c", "d", NULL}, and the value of argc would be 5.