A good video on how to use the rectangle tool enabling you to draw a box which could represent lumber or plywood, etc.
Next you will need to turn that box into a component. See this video...
Remember these steps when making a component (I prefer components over making a group): triple click an object to select all faces and lines of the object (remember 1 click selects the face of the object, a double click selects the face and the bounding box and triple click selects all of the object). Then the keyboard shortcut to make the object a component is "G". Give the component a name such as leg, or top for example. To return a component back to loose geometry, select the component, right click it and select explode.
In both videos, the user is using keyboard shortcuts. Learning keyboard shortcuts makes modeling much easier especially learning all that the mouse does. See keyboard shortcuts here: