The WhatsApp icon looks like a white telephone in a green speech bubble. You can find it on your home screen or in an app folder. Scroll down your Chats list, and tap a chat to open the conversation. If WhatsApp opens up to a chat conversation, tap the {"smallUrl":"https:\/\/www.wikihow.com\/images\/1\/11\/Android7expandleft.png","bigUrl":"\/images\/thumb\/1\/11\/Android7expandleft.png\/30px-Android7expandleft.png","smallWidth":460,"smallHeight":460,"bigWidth":"30","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of an Android icon.\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} button on the top-left to go back to your Chats list. This will highlight the message, and your options will pop up on a menu. This will copy the selected chat message to your iPhone's clipboard.  You can now paste the copied message to another chat or any other text field, such as your Notes app or a web page. To paste the copied message, tap and hold any text field on your iPhone, and select Paste on the pop-up menu.

Summary:
Open WhatsApp on your iPhone. Tap the chat you want to copy a message from. Tap and hold the message you want to copy. Tap Copy on the pop-up menu.