1. OpenFileDialog Class is available in System.Windows.Forms namespace. So, you need to add below line in your code.
using System.Windows.Forms;
If you are still getting error, then you might need to add reference to System.Wondows.Forms.dll, which can be done following this step: Right Click on Reference folder in Solution Explorer. In .NET tab of the Add Referrence Dialog box, Search for System.Windows.Forms and than Click on ok button.
2. Second Error should also get resolved by following above steps.
3. For Error no 3rd to 11th mentioned by you, is coming because you have not declared any object of such names. like for resolving error no 3, you need to create an object named openFileDialog of class OpenFileDialog as below.
OpenFileDialog openFileDialog = new OpenFileDialog();
Hope this will help. !! Happy Programming !!