If you want to store application configuration settings, you can leverage the 'AppSettings' in the App.Config or Web.Config file depending on if its a windows or web application.
I believe the syntax for reading the configuration values is
string configValue = Configuration.AppSettings["ConfigValueName"];
The configuration file will look like this
<configuration>
<appSettings>
<add key="ConfigValueName" value="ABC"/>
</appSettings>
</configuration>
With probably lots of other stuff.
If you need to store information about users or other repeated entities in your system, you will need to build a database and write code to persist / read data to / from the database.
You could make a class serializable and automatically serialize it to XML or Binary, OR you could use a SQL database. There are many .net technologies for accesing databases just look up ADO.net , LINQ and the Entity Framework.