Firstly you can buy some books, get some video tutorials or get some ebooks (you know where to get right? =P) then start learning the basics...

Secondly you can search for some tutorials over the internet... http://www.good-tutorials.com <-- there're a lot here... but if you learn in this way, don't just follow the steps blindly, try figure out what every step does, and you'll learn a lot of photoshop tricks...

Thirdly you can try learning with trial and error... but in this way you might learn very slow (very impressive though, you won't forget easily)...

Lastly, you can get a photoshop course with some bulks... but i dunno where you can get it... =P