How / When do I quote another member's post in my reply?

First of all you need to be a registered member to post in the site. If you haven’t registered yet, you can do so by clicking the link here. If you are already registered, use the prompt at the top right portion of the forum page to login.

While you reply in a message thread, you can quote another member's post by clicking on the “Quote” button that you see at the bottom right porti on of every post. It will automatically open a new message window, with the other member's post quoted. You can then add your own response below it in the same window and submit.

Note: It is not always necessary to quote another member's post when you post a reply. Do it only if necessary, like you are responding to a member who had posted long back in the thread, then you want to quote their original message and respond. If you are responding to a recent message it is not required to quote. You response can be related well.

Otherwise, just to post your reply only in the message thread, use the 'post reply' button seen at the top and bottom right of the message thread.

Refer this to know more about posting a reply to an existing thread.
Refer this to know more about starting your own message thread.