Note: Turtle Blocks JS closely parallels the Python version of Turtle Blocks, the version included in the Sugar distribution. Sugar users probably want to use Turtle Blocks rather than Turtle Blocks JS.
Once you've launched it in your browser, start by clicking on (or dragging) blocks from the Turtle palette. Use multiple blocks to create drawings; as the turtle moves under your control, colorful lines are drawn.
You add blocks to your program by clicking on or dragging them from the palette to the main area. You can delete a block by dragging it back onto the palette. Click anywhere on a "stack" of blocks to start executing that stack or by clicking in the Rabbit (fast) or Turtle (slow) on the Main Toolbar.
The basic buttons and basic blocks are explained in detail in Documentation.
A guide to programming with Turtle Blocks is available in Turtle Blocks Guide.
A quick start:
Google Code-in participant Jasmine Park has created some guides to using Turtle Blocks: Turtle Blocks: An Introductory Manual and Turtle Blocks: A Manual for Advanced Blocks.