well it's pretty simple.
let's store the string in an array:
char str[20]="This is a test string";
and store the result in some other array:
char result[10];
then use a for loop to detect when does the first "space" comes. The string of characters which comes before the first space will be the first word.
for(i=0;str[i]!=" ";i++)
{
result[i]=str[i];
}
the for loop exits when the condition str[i]!=" " becomes false.
so your result is stored in result.
Hope you understood how it worked which is more important!