input.txt (tab delimited fields)
--------------------------------
Name	Surname	Age
AAA	aaa	20
BBB	bbb	25
CCC	ccc	30



nawk -v col="Surname" -F'\t' '
   {
      if (NR == 1) {
         for (i=1; i<=NF; i++) {
            if ($i == col) { colnum=i; break; }
         }
         if (! colnum) {
            print("Specified column does not exist!");
            exit;
         }
      } else {
         print($colnum);
      }
   }
' input.txt



Output
------
aaa
bbb
ccc