This sounds like homework, so I won't give you code.
But it's not complex:
Handle the
ComboBox.SelectedIndexChanged event[
^] and get the new value.
Convert the new value to a number of seconds, and add it to the current DateTime.
Save this in a class level variable alarmTime.
Set your timer to fire once per second.
In the Timer.Tick event, compare the alarmTime and the current time. If it is now after the alarmTime, set alarmTime to DateTime.MaxValue and issue your reminder.