How to set values for auto-implemented properties

 I have a couple of questions if you don't mind.I know they are pretty basic but youare familiar with about where I am right now. I have created multiple classes and auto-implemented the properties. I need to use the Rates class like this...

class Rates
{
   public  int Days {get; set;}
   public decimal DaysRate {get; set;}
   ...
}
First, how do I get the text (or digits) from the textboxes to this property, such as Days should be equal to the text parsed from the daysTextBox.Text and DaysRate should be equal to the text parsed from daysRateTextBox.Text...and so on...
I need to multiply these two and have the answer accessible to the Charges class which is set as so...

class Charges
{
   public decimal DaysCharges {get; set;}
   ....
}
1) Where do the calculations take place..in the class form or on the Form1.cs or where???
2) How do I access information from one class to another?
3) Can I access multiple classes to perform calculations?

Okay....that's more than a couple. I have begun reading on creating backing fields but I do not know if this is in the right direction or not. Please forgive any ignorance I may have but this way (creating classes and auto-implemented properties) seems more"bulletproof" than my initial attempts at this program.

This is the link to the shared folder. Thank you for your comments. Here is the link for those of you who have any insights as to how I should tackle this problem. This is the new link for the updated program...take a look and see if you have the time to answer back to me.....thank you.