Here's the basic controls:
Left mouse: Places the 3D cursor, and drags the movement arrows of whatever you have selected.
These arrows for translating objects can be exchanged for rotation and scaling, using buttons at the bottom of the 3D view.
Right mouse: Selects and deselects.
Middle mouse: Rotates view
A: Deselects everything if anything is selected, selects everything if nothing is selected.
Shift-A: Brings up a menu for creating an object at the 3D cursor.
X/Delete: Deletes the selection.
Tab: Toggles the 3D view between object mode and edit mode.
Object mode allows you to adjust the scene as a whole.
Edit mode allows you to edit the vertices of an individual object.
There are other modes too, which can be found with a pop out list at the bottom of the 3D view.
Spacebar: Brings up a search for all the functions you can use in the 3D view.