Фиксированный код (с моими комментариями):
choice = int(raw_input("Enter 1 to enter sec. Enter 2 to enter mins")) # no need to use float here, also in python2 please use raw_input
if choice == 1: # you tried to compare `choice` string with 1 (always False), compare choice variable value with 1 instead. Also note, that you missed : after if
a = float(input("Enter secs"))