You can reach more drawing tools by using "combo" buttons. These buttons have a black triangle in their lower right corner.
For complicated molecule-drawing you can use the Drawing tool , which enables you to create bonds without changing tools, or even releasing the mouse button. (See details here.)
You can find more functionalities in context menus. You can reach context menus by right clicking on certain parts of your structures (for example atom, bond, R-group definition etc.).
View Settings give you the freedom to see the molecule as you want to. You can set valence check on/off, display options such as ball and stick, and much more useful view options.
You can get more information about the other features of Marvin JS from the Video tutorials.