HealingMe gave great examples. There are a lot of different ways to quote other peoples posts. The easiest way for me is to hit the 'reply' button at the bottom of someones post (). It takes you to a page where it automatically puts their post at the top. If you type below the text that's there it will quote the whole post. If you want to just quote part of their post then delete the text that you don't want but make sure you don't remove the brackets that say [ quote] and [ /quote]. These are the codes that create the little box around the post

If you wanted to quote multiple posts, as HealingMe said you can use the button next to it () to click all of the posts you want to quote and then click 'add reply' near the bottom of the page (). It will then take you to a screen where all of the posts are shown. All you need to do is click between each one of the posts and write your text there.