A little help:
Duplicate: Shift-D
Extrude: CTRL-Click
Join objects: CTRL-J
To create a new object: hold a right click until a menu appears:
add>mesh> then select what you want to add, and then it will prompt you asking you how many vertices you want in it, and stuff like the size.
IMPORTANT: For Conversion into g3d format, all faces need to be triangles. To achieve this: go into edit mode, select everything(A), and then CTRL-T.