Hopefully you are making some progress on your assembly, but I guess that we can start with the tubing. I would focus first on making the individual parts. Create as many as you can right now. Are you familiar with configurations?
From looking at your pictures, on the assembly side I would familiarize myself with the concentric mate first (reading & playing). you mentioned you needed help with mates, so sorry if I am being too simplistic. The concentric mate is going to make circles, round extrusions, share a center point or axis. This will make your tube slide within another tube.
How far have you gotten on your parts?