Annibale,
I agree with Im: you need to get a book or use some of the helpful Photoshop tutorial websites such as :
Cambridge in Colour - Digital Photography Tutorials (yes, I know it says colour but the principles are the same and it is user friendly)
How To Create Black And White From Color - Color Effects | PhotoshopSupport.com
Adobe Photoshop and ImageReady Tutorials and Techniques
But to get you started, when you've loaded your image into Photoshop, pull down the Image menu, select Mode and then select Grayscale. This will provide a black and white image from a colour image and you can then experiment with Levels, Curves, Dodging and Burning until you reach an image to your taste.
Pete.