如何从json格式的restful-API获取用户名和密码到laravel 5.4？

i don't have more experience in API. i am using laravel 5.4 version. where i self create registration API into Restful-API. i am sending request into android phone but can't get its value into laravel5.4. so please tell me how can i get it. with explanation ... Please Note that user name and password is into json format.. sample is give below...

{
    "arg": {
        "username": "410",
        "emailID": "password",
        "phoneNumber": "+91XXXX98"
    }
}