Welcome to the Forum!

I agree completely with Alex, have a good read through the build threads before you start anything and familiarize yourself with the do's and don'ts and see how other builders have done it.

Most newcomers tend to go for basic pushy bricks so kudos for deciding on an active weapon, not entirely sure from you description what you mean so i'll be interested to see how it works.

In terms of the actual robot, will it be for full combat? i.e spinners, or is it just for the non spinner events. The reason i ask is this will dramatically change how you need to construct the robot and how well to armour it. If it's the latter you can get away with 10-15mm hdpe with barrel nuts and it will last as long as you need it to providing you build it well, for full combat you really want to aim for 15mm minimum, my robot this year was all 20mm hdpe and although it now looks a bit torn up it hasn't taken any damage that would mean i have to rebuild it. You'll notice that me an Alex both suggested barrel nuts, they are the best way IMO of joining plastics together and make a very strong join that will survive most spinner impacts.

Have a look around, ask as many questions as you can and i look forward to seeing what you come up with.