Creating and using rest api in magento 2 is very easy but for that you need some startup example.Magento 2 uses token based rest api.First you need to authenticate user and get the token from magento 2.After getting token you have to pass this token to every request you performed.For example to get product details by SKU you need to first authenticate user and get token now pass this token to your  header to call web service  /V1/products/:sku

Here I have created example for get product by SKU using magento 2 rest api.Here is list of API that supported by magento 2. LIST OF Methods REST API MAGENTO 2

Please note that you need to create web service user/role it is not a good idea to use admin user into webservice.See here for how to create and use web service user in magento 2

Magento 2 REST API Authentication

//Authentication rest API magento2.Please change url accordingly your url
$adminUrl='http://127.0.0.1/magento2/index.php/rest/V1/integration/admin/token';
$ch = curl_init();
$data = array("username" => "wsuser", "password" => "password123");                                                                    
$data_string = json_encode($data);                       
$ch = curl_init($adminUrl); 
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");                                                                     
curl_setopt($ch, CURLOPT_POSTFIELDS, $data_string);                                                                  
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);                                                                      
curl_setopt($ch, CURLOPT_HTTPHEADER, array(                                                                          
    'Content-Type: application/json',                                                                                
    'Content-Length: ' . strlen($data_string))                                                                       
);       
$token = curl_exec($ch);
$token=  json_decode($token);                                                

Get Product By SKU REST API Magento 2

//Use above token into header
$headers = array("Authorization: Bearer $token"); 

$requestUrl='http://127.0.0.1/magento2/index.php/rest/V1/products/24-MB01';
//Please note 24-MB01 is sku

$ch = curl_init();
$ch = curl_init($requestUrl); 
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers); 
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);   

$result = curl_exec($ch);
$result=  json_decode($result);
print_r($result);