Magento API: Publish a new method in soap V2

I'm new in Magento and I'd like to create a my own API v2 method. I've built a simple project...

Mycompany
    Mymodule
        etc
            api.xml
            config.xml
            wsdl.xml
        Model
            Api
                V2.php
            Api.php

These are the main files...
(1) api.xml

<config>
    <api>
        <resources>
            <mymodule translate="title" module="mymodule">
                <title>mymodule</title>
                <model>mymodule/api</model>
                <methods>                    
                    <myapimethod translate="title" module="mymodule">
                        <title>myapimethod</title>
                        <acl>mymodule/myapimethod</acl>
                    </myapimethod>
                </methods>
            </mymodule>
        </resources>
        <v2>
            <resources_function_prefix>
                <mymodule>mymodule</mymodule>
            </resources_function_prefix>
        </v2>
        <acl>
            <resources>
                <mymodule translate="title" module="mymodule">
                    <title>Mymodule</title>
                    <sort_order>2000</sort_order>                    
                    <myapimethod translate="title" module="mymodule">
                        <title>myapimethod</title>
                    </myapimethod>  
                </mymodule>
            </resources>
        </acl>
    </api>
</config>

(2) wsdl.xml

<?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns:typens="urn:{{var wsdl.name}}" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.