Robocraft builds can be complicated. Here is a basic tier 1 platform that allows many guns and high mobility and all for free without having to go premium.
Build a 7 X 10 base and then a few blocks on the sides for your weapons. Steering whieels in the front and additional wheels towards the back.
From here you can add up to 7 weapons to your Robocraft, possibly more and still have high mobility. This is good for your first few games as you build up RP you can start adding higher armour and move up the tiers of Robocraft!