The easiest thing to do is to place your cursor (the little arrow on the screen that indicates position) at the beginning of what you want to quote, push the left mouse button down, and - holding the button down - swipe along the screen until you get to the end of what you want to quote - aka, highlighting the text. When you stop and take the pressure off the left mouse button, a small dialog box will appear that says +Quote and Reply. Hit quote and a <snip> of what you highlighted will be saved. Then when you start a new message in the box at the bottom of the screen (where it says "Write your reply...") you will see an option at the bottom to Insert quotes. Click that and you will see all the quotes you have saved. Mouse click on the one you want and it will be inserted in your new message.