Hi mate,
I found it easiest to start off just making a nice working drive system, and forget the weapon for the moment that can be added later on.
I found trying to make a whole robot first gave me too many problems to dort out, whearas just a good driving robot at first let me do things step by step and so i could work out what went wrong and why withought getting muddled up and confused.
This is how i found it easiest, but its upto you at the end of the day, and whether you are already familiar with the workings of robots.

Good luck with it and keep intouch with how its getting on!
Grant