I guess the first challenge is doors... The theme is "doors" -- so do with it what you will ;-) Take as many photos as you need to/can in order to explore the theme fully and the trick is, don't delete anything initially! Part two of the challenge is to really consider why you took a certain shot, what inspired you, what moved you to capture, what statement are you making, what story are you telling, etc. Some of your photos may have natural captions or titles that are born simultaneously to capturing the image and others may remain nameless...both are important. But learn to think and document this journey in images first and words, thoughts, and ideas second ;-)
So there you have it -- turning you loose and can't wait to see what your eye sees and unveils ;-)