Just press enter to talk to the enemy during combat.
Choosing your opponent is a little more complicated. First you must add them to your list of "friends", to do this you need their "Code," which is a 3 digit number in the lower right-hand corner of their screen next to their username. Click on your code and enter their username in the username spot under the "Add user" section, and their code in the place to the right of it. Then click the "+," button. Their name should appear above.
Now that you have added them to your "Friends" list, you can chat with them by clicking their username, or challenge them by clicking the crossed swords beside their name.
My username is catfishperson and my code is 111. So you can practice with me if you want.