How can i learn photoshop for web designing?

yes, you can simply use Internet for learning Photoshop

There are many lessons and samples for teaching and making some activities by photoshop

you can start searching on google for photoshop lessons

another thing, every photoshop program has a help menu you can see the whole help and learn many things, also some times there are some tutorial lessons you can benefit from as good samples

Go Online, they got much to offer from Tutorial Videos to Step by Step Teaching...

www.photoshop.com/tutorials

www.tutorialized.com‎

best website for learning graphics desining

http://psd.tutsplus.com/

Go to adobe.tv and learn a lot of web design techniques.. but you must use Adobe Photoshop CC because it is the only Photoshop CC that can generate CSS and paste it on Adobe Edge code... you can make a web desing using Adobe Photoshop as long it is CC.

I would suggest Try Lynda.com or Udemy.com with lot of practice. 

You should invest in membership of Lynda.com -- the best tutorials I've found online. Once you learn the basics, move on to tutsplus.com for advance things.

simply look for it on youtube, you need to learn the web design concepts first then learn how to use tools to slicing then cooding and finally add UI Effects

Either from You Tube or any designing Institute.