The easiest way would just be to do situps in the morning. If you do 50/60 when you wake up, before you do anything, it wont make you too tired and you should start to see some improvement in a few...
I'm not sure what you are asking with this question - I do not know what you mean by "through exercise". Could you please ask your question again and be more specific.
YES!!!! exercise makes your mind think. It is always good to ride your bike before doing homeowrk or paper work cause it gets your mindstarted and ready to do work.