I started on Magento2 Custom rest API module creation
for that i follow the below post

API Module for Magneto2

url:- http://localhost/magento2a/rest/V1/calculator/add/1/2

app/code/AlanKent/CalculatorWebService/etc/module.xml

<?xml version="1.0"?>
<!--
/**
 * @copyright Copyright (c) 2015 X.commerce, Inc. (http://www.magentocommerce.com)
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/Module/etc/module.xsd">
    <module name="AlanKent_CalculatorWebService" setup_version="2.0.0"/>
</config>

app/code/AlanKent/CalculatorWebService/etc/di.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../lib/internal/Magento/Framework/ObjectManager/etc/config.xsd">

    <preference for="AlanKent\CalculatorWebService\Api\CalculatorInterface"
                type="AlanKent\CalculatorWebService\Model\Calculator" />

</config>

app/code/AlanKent/CalculatorWebService/etc/webapi.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<routes xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../../../../../app/code/Magento/Webapi/etc/webapi.xsd">

    <!-- Example: curl http://127.0.0.1/index.php/rest/V1/calculator/add/1/2 -->
    <route url="/V1/calculator/add/:num1/:num2" method="GET">
        <service class="AlanKent\CalculatorWebService\Api\CalculatorInterface" method="add"/>
        <resources>
            <resource ref="anonymous"/>
        </resources>
    </route>

</routes>

app/code/AlanKent/CalculatorWebService/Api/CalculatorInterface.php

<?php

/**
 * Copyright 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace AlanKent\CalculatorWebService\Api;

use AlanKent\CalculatorWebService\Api\Data\PointInterface;

/**
 * Defines the service contract for some simple maths functions. The purpose is
 * to demonstrate the definition of a simple web service, not that these
 * functions are really useful in practice. The function prototypes were therefore
 * selected to demonstrate different parameter and return values, not as a good
 * calculator design.
 */
interface CalculatorInterface
{
    /**
     * Return the sum of the two numbers.
     *
     * @return int The sum of the numbers.
     */
    public function add($num1, $num2);
}

app/code/AlanKent/CalculatorWebService/Model/Calculator.php

<?php

/**
 * Copyright 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace AlanKent\CalculatorWebService\Model;

use AlanKent\CalculatorWebService\Api\CalculatorInterface;

/**
 * Defines the implementaiton class of the calculator service contract.
 */
class Calculator implements CalculatorInterface
{
    /**
     * Return the sum of the two numbers.
     *
     * @api
     * @param int $num1 Left hand operand.
     * @param int $num2 Right hand operand.
     * @return int The sum of the two values.
     */
    public function add($num1, $num2) {
        return $num1 + $num2;
    }
}

it work fine for me

Guys if you get error

 [2015-10-14 09:24:54] main.CRITICAL: exception 'InvalidArgumentException' with message 'Report ID: webapi-561e0346e13a1; Message: Each getter must have description with @return annotation. See Magento\Catalog\Api\ProductTypeListInterface::getTests()' in D:\xampp\htdocs\magento2a\lib\internal\Magento\Framework\Webapi\ErrorProcessor.php:194

then make sure in Interface file where you declare the function above that there should be @return in comments else it will through error

that is getTests its my new method aove that i have comment with @return if you missed the it will through the error

<?php
/**
 * Product type provider
 *
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Catalog\Api;

/**
 * @api
 */
interface ProductTypeListInterface
{
    /**
     * Retrieve available product types
     *
     * @return \Magento\Catalog\Api\Data\ProductTypeInterface[]
     */
    public function getProductTypes();

    **/**
     * Retrieve available product types
     *
     * @return
     */**
    public function getTests();
}

it look wired but its works
above is the solution for Error

main.CRITICAL: exception 'InvalidArgumentException' with message 'Report ID: webapi-561e0346e13a1; Message: Each getter must have description with @return annotation. 

Note:- once module is pasted dont forget to run upgrade command that is D:\xampp\php>php D:\xampp\htdocs\magento2a\bin\magento setup:upgrade

(in local system i have php in D drive so change as per your system)