I am doing the specifications using Salesforce API I searched so much but didn\'t find the list of fields that i could retrieve using GET method Someone could provide me the
You need to use a "describe" call. It'll return a list of all objects your user can see in the org and it'll include links how to get more detailed info about all fields on say Account. (you can jump straight to describing an Account if you want, the links are constructed in a predictable pattern)
"Describes" are limited to what your Profile can see so if the profile lacks the "Read" checkbox on some object - it won't appear in the results.
Go to https://workbench.developerforce.com for example and experiment (from menu select Utilities -> REST explorer)
/services/data/v41.0
/services/data/v41.0/sobjects
/services/data/v41.0/sobjects/Account/describe