all you need to know to design parts in blender:
G - move, + x/y/z to bind to only move in that axis.
B - box select
A - select/deselect all
shift S - cursor options, use it to move your cursor (that round widget that controls where parts spawn and can be used as a reference point for other operations.
E - extrude. self explanatory + x/y/z to bind movement to one axis
P - separate, as in select a set of faces or vertexes and split them off to a new object.
hold shift instead of ctrl for multi-select.
Tab switches between object and edit mode
default unit is an arbitrary "blender unit" simplest way to design for printing is to treat one BU as one MM. for instance, if you want a part 40 mm tall. select the vertexes you want at the top, and type G (or E), Z, 40, enter.
there are a bunch of more complex things you can do with the cursor, and many more keyboard shortcuts, but this set should be enough to get you actually into the program without being totally lost. its approach is a bit different from traditional cad packages, but once you learn a bit about the workflow, it becomes natural and intuitive to you.
edit: wow! forgot basic view controls.
numpad 7 - top view
numpad 1 - right view
numpad 3 - front view
numpad 5 - switch between orthographic view (for designing) and perspective (to see what it looks like in reality)
the other numpad keys move the view in increments between the top/right/front/left view, experiment to see whats what, i dont pay attention to them much. lol
