Sketches not fully defined and I... admittedly don't understand constraints yet.

Most dimensions will be driving as you use them to define the shape you want. Occasionly a dimension may be added to show a dimension which is already defined but helps read ability. E.g. The dim from one side to a feature is defined as is the overall length, and this makes sense from a modeling perspective, but you put a driven dimension from the feature to the other edge because it saves having to subtract the driver from the overall length every time.