basic OOP questions panicked student need help plz!

im having issues with a program im trying to finish. Its a simple mortgage payment program where i use different classes and all that. I have instantiated the classes fine and can pass the value back out of the derived class back to the base class. My problem right now lies w/ the passing the values into the derived class by properties. I have 3 values (2 integers and one double for interest rate). I have to use a constructor where i dont kno what i need to put in there,guess im supposed to pass the values in this. Then have 3 properties and i dont know if they should be public or private or protected or shared, but i think i need both get and set right? If anyone could please help me out i would be very grateful this thing is due tomorrow and i have about 20 hrs into it! thanx in advance