Where I can find the pieces needed in a set?
If you don't physically have the instructions, you can look up the set ID on LEGO's Customer Services page dedicated to providing building instructions.  

The inventory list is usually found right after the final building step. For #7946 the parts list can be found on pages 74-75.

If for whatever reason you cannot access the instructions from the LEGO site, perhaps the file has been removed or you're unable to view PDF files you can still find a part list on sites like Bricklink and Brickset (see parts tab).