There are two ways in. With primitives you stack boxes, cylinders and spheres until the shape is right — fine for a mount, a spacer or a bracket. For a profile that standard shapes won't give you, draw a sketch first and pull it up (extrude) or spin it around an axis (revolve); a knob, a wheel or a bottle appears in a single move. Model at full size in millimetres from the start: a 3D model carries no unit of its own, and your slicer will later assume one unit means one millimetre. Everything runs in your own browser, so your design stays on your machine.
You don't make a hole with an eraser; you subtract a cylinder from the model — a boolean. Need a row of fixing holes? Subtract one and repeat it with a pattern: quicker, and the spacing stays exact. Then measure, because the number in your sketch is not the size that comes off the printer. A 5 mm hole for an M5 bolt will bind; count on two to four tenths of a millimetre of clearance. Unsure about a thin wall? Check the stress with finite elements before you burn hours of print time.
Export STL for the printer: the file holds nothing but triangles, exactly what a slicer needs. If someone else still has to change the model, STEP is the better choice — it keeps the real shape, so a colleague or a supplier can carry on with it in their own CAD package. OBJ is for models heading into a render or 3D program. Watch how fine the triangles get: the rounder the shapes, the more triangles and the bigger the file. Too big for a mail attachment? Send it as a download link instead.