Help Putting Contents of XML file into C# textbox?

Question

What I have is a somewhat 'Money tracker' were you enter in an amount of money, it's added to a string, the string is put into a text box, and it's supposed to stay there even though I close the program.
That didn't work.
So I thought about putting textBox4.Text (Or the double) in between two <amount> tags in XML.
How would I go along doing this?  Also, how would I get it to reload the content of the <amount> tags into that text box next time I upload it?
Thank you for your time.