A large square or rectangular box is generally the easiest to build. Their sharp corners can be hazardous, though. If you have children playing in your yard, you may wish to soften corners by rounding them off. Also, these beds can have a rigid, formal countenance that may be a little too Spartan for some tastes. Follow the contours of the land with gentle curves or create your own configuration.
Curves are more inviting than right angles, but they are more difficult to lay out and construct.
An ordinary garden hose can be used to form a template on the ground. This makes it easier to be creative without having to build something, take it down, and start over again. Spray paint can be used to mark your final design. The width of your beds should be tempered with practicality. A major advantage of raised beds will be lost if your bed is too wide.