An example about how to consume a Salesforce api rest, for instance:

/services/apexrest/

The example shows how to authenticate using username and password to get the access token and then how to do a request to this Salesforce web service:

      
@RestResource(urlMapping='/hellorest')
global class HelloRest {

    @HttpPost
    global static String echo(String name) {
        String echo = 'Hi, '+name;
        System.debug('Echo: '+echo);
        return echo;
    }
}
      
    

See the code from Github https://github.com/andrescanavesi/salesforce-apex-rest