C# using WPF to enter data

I want to write a C# WPF application using visual studio 2010. I want to be able to allow the user to do the following:

1. enter some data,

2. select a file from a directory path,

3. error message will be displayed if the user enters wrong data,

4. the user will hit the submit button and call a windows api.

Thus can you tell me how to accomplish this goal?