How do you highlight a bit of a post to quote 
I only know how to 'quote' the whole post; I then delete the parts I wasn't wanting to include in my 'quote'.
When you are looking at the post you want to quote some of, look at the right hand side of the screen above the post. Click where it says
Quote and it will create a reply with that post quoted in it. You then delete any of the quoted material you don't want and add your own comments, and post.
Take care when deleting part of the quote that you don't delete any of the text enclosed by [square brackets] - the line starting [quote author and ending with a lot of numbers and a closing square bracket] starts the quote and the line that is just [/quote] ends the quote. If you think you might have messed it up you can use Preview to see what your reply will look like.
You can also include a quote when you are writing your reply. Scroll down the screen and you will see the latest of the previous posts listed there. At the right hand side under the date and time of posting you will see
Insert Quote. Click there and it will insert that post as a quote in your reply.