To build things you need multiple prims. 
How to join two prims together to form a single object.

1°) Rez two prim on the ground

2°)  Go into  build mode by clicking ( Build ) on the bottom row of buttons in the Gui (graphique interface)

3°) Keep SHIFT key pressed down and click on all of the prims you want to join 
     all will be highlighted yellow once you click

4°) Once all are highlights yellow, press CTRL+L to link the prims.
   Sometimes the server lags
   but 1 prim, the last one you click will turn blue. otherwise, continue to press CTRL + L until all your prims are linked