haha Ssquir is right allot of people tend to focus on apples or shapes to get used to blending ect http://itchstudios.com/psg/art_tut.htm#light_stuff heres a nice tutorial about lighting that can help with this. Avoid using texture brushes for a little and just get used to photoshops flow.
I like the interesting shape you have to the building with the plus shaped base, there are some things a straight edge could help polish up, but the concept is very cool!
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good start tho, dont get frustrated too much if its not exact.
Best of luck!
http://www.ctrlpaint.com/ <-- maybe useful to you
http://www.youtube.com/user/FZDSCHOOL?blend=1&ob=video-mustangbase <-- and i find fen really useful even if you can't follow what hes doing the things he talks about are important and core.