1.Login to admin panel.
2.Click on System.
3.Click on User Role.
4.Click on User Role “Add New Role”.
5.Set Name as something like “Web Service Role”.
6.Click on “Role Resources”.
7.Set only those resources that are required for web service.
8.Click Save Role.
9.Now create user for the role we have just created.
10.Click on System.
11.Click on “All Users”.
12.Click on “Add New User”.
13.Set users fields like User Name,First name,lastname, password etc..
14.Click on “User Role” Tab.
15.Select the newly created role.
16.Click “Save User”.
17.If it ask for clear cache then clear it.