There is no easy answer to this. You need to learn each tool and what it does, and there are a lot.
I would recommend buying a book and working through the book.
The "for Dummies" range of books are a really good starting point. There is one for each version of Photoshop. Here is the one for the latest version:
You will also find tutorials on the Internet, but I would recommend a book as a more complete learning method. You could also ask your local college if they have a course for Photoshop - they may do a day course. Adobe have courses, you can do these online or you can attend a class. See here:
Finally, you can get instructional DVDs. Try asking your local photography or art shop if they carry them. There are also online videos such as www.lynda.com
. If you can't afford to subscribe, try looking on Youtube.
NOTE: these may be quite advanced videos in some cases. You really need to learn the basics first