“Great! Now I have to re-do it.”
While you're writing a document on your computer, the program crashes and you lose all the work you've done. You are upset. You say this to yourself.
Great! Now I have to re-do it.
People often use the word "great" to mean the opposite of great - to mean that you are upset about something. This can also appear as the phrase "That's just great."
One way to use the word "now" is to express the result of some situation or action. In the example above, the software crashed. Saying "Now I have to re-do it" means "As a result of the software crashing, I have to re-do it.
As another example, if the TV show you watched every week has finished its last episode, you can say:
Well, now I have my Tuesday nights free.
To "re-do" something means to do it again because there was a problem the previous time you did it.