36 bytes

In this problem, the key observation is that the ASCII value of each letter is equal to its position, offset by ~64~. A basic implementation might read the character via scanf() and output its ASCII value, minus ~64~:

#include<stdio.h>
int c;int main(){scanf("%c",&c);printf("%d\n",c-64);}

This is pretty concise, but we can do a lot better. Abusing C's implicit int rule allows us to remove the type declarations, at the cost of a few warnings. This yields a ~63~-byte solution:

#include<stdio.h>
c;main(){scanf("%c",&c);printf("%d\n",c-64);}

Also, scanf() is rather long to simply fetch the first character. We can instead use getchar(), which does exactly that. This saves us another ~9~ bytes:

#include<stdio.h>
main(){printf("%d\n",getchar()-64);}

Finally, to obtain the ~36~-byte solution, we realize that the #include header is unnecessary, as C allows the declaration of most library functions without needing their headers:

main(){printf("%d\n",getchar()-64);}