If you want to open your internet browser at Yahoo.com when you click the Mail icon:
1. Go to Yahoo.com.
2. Select all of the URL (
http://www.yahoo.com), click and hold, then drag to the desktop. There should be an icon on your desktop with Yahoo.com as it's name. You can close your browser now.
3. Command (apple button) click the Apple Mail icon in your dock.
4. A new Finder window should open with the application selected.
5. Press Command+I to get info on the program.
6. On the top left of that Info window, you'll see a small icon. Click it and press Command+C to copy it.
7. Close all open windows.
8. Select the icon on your desk that you made in step 2 and press Command+I to get info.
9. Select the small icon at the top left and press Command+V to paste Mail's icon over it.
10. Drag the "real" Mail out of your dock, you'll see a poof when you let go.
11. Place the "Mail" icon in your dock, to the left of the Trash.
Now when you click that icon it will take you to Yahoo.com.
Hopefully, this is what you were asking...