How to get user data when I provide the uid or username - Services
URL to fetch the user information by providing uid as follows.

http://[yourdomain]/[endpoint]/user/[uid]


Example: http://www.example.com/rest/user/2 will fetch user information, having user id 2. 

You can get the user data by providing username, you can use pass name (username) as a parameter to the service URL like follows

http://example.com/&lt;endpoint path&gt;/user?parameters[name]=krishna


If you have enabled session authentication you might have to pass X-CSRF-Token in headers.
For session authentication you can find my answer here