Press the speech bubble icon that has a “quote whole post” tooltip when you hover the mouse over it. What gets quoted depends on whether you clicked the generic reply button at the bottom of the thread (in which case it will quote the last post), or whether you clicked reply on a specific post (in which case it will quote that post).
You can then edit the quote down to just the parts to which you are replying. Note that if you try to quote a whole post, the forum software will automatically remove it again since there’s not much point in everyone quoting whole posts.
Alternatively you can copy and paste some text and then click the quotation marks button to blockquote something manually, which will do the same job but wont have the original author username embedded in the quotation.