Test case for a switch statement C#

I'm a quite new person for C# and unit tests. I need to write a unit tests for a switch statement and I have to admit that I already have been through many of pages trying to find something. Does anyone could give me any tip how to create it? please My code below:

public static Message create(String body)
{
    Message result = null;
    switch (body.ToArray()[0])
    {
        case 'S':
            //checking the regex for a sms message
            Match matchS = Regex.Match(body, @"[S][0-9]{9}\+[0-9]{12}");
            if (matchS.Success)
            {
                MessageBox.Show("This is SMS message");
                //if the regex will match a sms_message class will be started
                result = new Sms_Message(body.Substring(1));
            }
            //if the regex doesn't match the message should be displayed
            else throw new Exception("I don't like content!!!!!!!!!!");       
            break;
        case 'T':
            //checking the regex for a tweet message
            Match matchT = Regex.Match(body, @"[T][0-9]{9}");                     
            if (matchT.Success)
            {
                MessageBox.Show("This is the Tweet message");
                //if the regex match the message should be displayed and the class Tweet will be started         
                result = new Tweet(body.Substring(1));
            }
            break;
        case 'E':
            //checking the regex for a email message
            Match matchE = Regex.Match(body, @"[E][0-9]{9}"); 
            if (matchE.Success)
            {
                //checking the content of the message by using function 'BodyIsSir'
                if (BodyIsSir(body))
                    //if function return true the SIREmail class will be started                                               
                    result = new SIREmail(body.Substring(1));                       
                else
                    //if function return false the StandardEmail class will be started
                    result = new StandardEmail(body.Substring(1));                  
            }
            //when regex will not match the text message will be displayed
            else throw new Exception("I don't like the content");                   
            break;
        default:
            //when the content of the email message will not match
            throw new Exception("I dont like first letter!");                         
    }
    return result;
}

private static bool BodyIsSir(string body)
{
    //checking the body of email message if this contain regex for checking the sort code
    Match matchSIR = Regex.Match(body, @"[0-9]{2}[-][0-9]{2}[-][0-9]");              
    if (matchSIR.Success)
        return true;                                                                
    else
        return false;                                                                   
}