When creating a test for api testing how do I determine which method I should call(how to know method name) and and its return type(GET,POST,etc)?
For example : Do I have to talk to developers and get methods names for the task that I want to test?
This depends on how your devs developed the particular service under test. If we're talking about REST architecture, then there is probably WADL file exists that describes the interface you may use for interaction.
There is also a number of frameworks which are widely used in modern world to build up REST services. When devs build up a service they describe it in some declarative language that is converted to the service code. Those files are used for building test clients. As the example of such the framework you can refer to SWAGGER framework or alternatives.
Otherwise you have to ask your development to provide you the full specification of the service interface.