Is yours a CL/GL/SE or GTI/VR6 Golf? I'm assuming you have a mk3?
If it's the first ones, then you will have stud fitment of 4x100, if the latter then 5x100.
The offset of anything from 20-45 is pretty much fine. The lower is the offset the further the wheels will stick out.
You don't really need to measure anything, as long as you know the specs of the alloys you want.
Also another thing to keep in mind is the centre bore of the wheels, because sometimes it can be larger, than what's on your car, in which case you will need spigot rings to match the bores of the car and the alloys.