var message = GmailApp.search('your search text', 0, 1)[0]
.getMessages()[0];
var messageDate = message.getDate();
messagebody = message
.getPlainBody();
That code block gets the first message of the first thread matching the search field. I can understand why it's somewhat confusing, but it's important to understand that, at least in gmail world, a thread does not necessarily have more than one message in it. Every message in gmail is part of a thread. Kind of like a directory or folder with only one file in it.
The messageDate line is just one example of what else you can do once you've identified the message. You can replace "PlainBody" with just "Body" if you want the rich text instead of plaintext.