Don't be afraid to play around, anything you do can be undone by pressing the undo button or control-z.

The brush tools lets you draw polygons, hold down shift to draw straight lines. You can add hinges between objects by using the hinge tool, and the Fixate tool lets you attach objects to each other or the background. 
The drag tools lets you drag around objects when the simulation is running. The brush tool instead moves the objects (try it to see the difference). Holding down control when moving will create a clone of the selected objects. Rotate objects by grabbing onto them with the right mouse button.

Grabbing onto the background with you right mouse button and moving the mouse will pan the view (ie move the camera). Use the scroll wheel to zoom in and out, or use the slider on the left.

You can select objects by clicking them, and select many by encirceling them with the brush tool or the box tool. You can add and remove objects to a selection by holding down control and/or shift while selecting. Selected objects can be assigned to groups by pressing ctrl-#, where # is 0-9 on your keyboard. Press that number to quickly select the same objects again. Pressing shift-# adds to the group and alt-# substracts.

Erase objects by drawing a straight line over them with the brush tool, or just select them and press delete.

Clicking an object with the right mouse button will bring up a menu where you can modify it's properties. The same goes for water. Clicking on sliders with the middle mouse button resets them to their default value.

Useful shortcuts:

B - Brush
D - Drag
P - Plane
C - Circle
X - Box
N - Chain
S - Spring
F - Fixate
H - Hinge
Ctrl-Z - Undo
Ctrl-Y - Redo
Delete - Delete selected objects
Space - Pause/Resume simulation