On some boards you might be able to post and reply as a guest user. But most communities require registration.
As a registered user you can go to a forum on a board where you have permission to view threads and leave replies. To reply you have a few options. You can click on the 'Post Reply' button and add a new post to the end of the thread. Alternatively, you can leave a quick reply in a quick editor box listed below the posts in the thread.
When using 'Quick Reply' you can choose to quote a particular post if you are replying to something someone wrote. You may need to click the quick reply button in a post to activate the quick reply box before you can type into it.
If you want to post replies to multiple posts you can select them by clicking the multi quote button . This button will change to indicate that you've selected it. Clicking post reply will then bring you to the full editor with all the posts quoted.