Assuming your using Visual Studio.NET....

1. Drag a simple button control onto your workspace - label it "Browse..."

2. Drag a special control from the toolbox called "openFileDialog" onto your
form - it doesn't have a visual representation like a button, but Visual
Studio will show it for you in a "tray" down in the lower area of your
screen. It will have a default name of "openFileDialog1" - just leave it
there for now.

3. Double-Click the "Browse.." button you created in step 1, which will
launch you into code view, inside of an onclick event handler for that button.

4. Now, inside of the event handler for the browse button, you will write
code which will reference the openFileDialog1 object.
 - specify the filter you want to use:
openFileDialog1.Filter = "Text Files (*.txt) | *.txt | All Files (*.*) | *.*";
 - specify the title you want user to see in the dialog box that you will
create:
openFileDialog1.Title = "Select a file to load:";
 - optionally, you can set a default file location to begin the dialog
browser within
openFileDialog1.FileName = "C:\\Program Files\\someText.txt";

5. Also, within the same event handler, you will finally add code that will
actually launch the dialog box. After the user interacts with the dialog box
and clicks ok, you will want to access the openFileDialog1.FileName property
again (as it will now have changed to whatever the user specified). Use that
FileName property to load up your richTextBox. In the following code, I
actually launch the dialog box within the "if" conditional clause, and
evaluate the result:

if(openFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
{
   richTextBox1.Load(openFileDialog1.FileName);
}