i want to learn photoshop script .

I would download the SDKs:

Photoshop javascript reference: Adobe Photoshop CC Scripting Guide.pdf,

Javascript Tools Guide: JavaScript Tools Guide.pdf

And I would strongly second Davide's book on Photoshop Scripting, which is one of the best condensed sources of Photoshop scripting right now.

Think up of easy Photoshop problem and make research on this forum how to solve it by scripting. Then another one and so one. It best method to start learning.

One of the people JJ mentioned, Davide Barranca, has a course in Photoshop scriipting, Photoshop Extensibility Development Courses

You need to be a Photoshop expert know some JavaScript programming get Adobe Scripting Documentation. Must have a strong foundation of Image editing. understand Aspect Ratio, pixels, resolution, file formats support an advance users you must be.

You need to be able to design procedure to perform the automation you want to program using Photoshop feature.  

Photoshop scripting support  plug-in is very limited all of Photoshop features only some of Photoshop features can be scripted.  However, all that can be recorded and inserted into Actions can be used in Photoshop scripting using Action Manager code.  You will need to install Adobe Plug-in Scriptlistener and be able to extract Action Manager coder it records and create script functions to use in you Photoshop scripts. Search foe the "Clean SL" Photoshop script it will help your at this task and make Action manager code in you scripts more readable.  The recorder code is not easy to read.

For Script Dialogs you need to use Adobe's ScriptUI support.

There is not many books about scripting Photoshop. So you will need to use this forum to get help with your scripts. There is no longer a separate scripting forum or a strong Photoshop scripting community on the web.   

The old forum content is available on line and this site search  has been improved some so you should use searching before asking for help.  You will get the information faster that way. There is a lot of knowledge and sample codes in the forum's data. 

Some of the best scriptors no longer participate here. They have left or passed away. But the knowledge they poster here is still valuable and useful.  Get to know the name of the Present and Past script experts Paul Riggott, Mike Hale, SuperMerlin, R-bin,  DBarranca, pfaffenbichler, Chuck Uebele. There are others these head my list of experts if I see the other name I'll know them. 

I only hack at scripting Photoshop still I think some of what I have posted can be useful to some users.