It's easy:
1) Open your projects Properties in the solution explorer, and double click on "Settings.settings"
2) In the resulting grid, change the Name to "MySetting", and set the Value to "Defaulted value". Leave Type and Scope as "string" and "User" respectively.
3) Save and close the settings window.
4) To read your setting:
string s = Properties.Settings.Default.MySetting;
5) To write your setting:
Properties.Settings.Default.MySetting = "My new setting value";
Properties.Settings.Default.Save();