DivX is probably the most commonly used codec for high resolution copies from DVD to CD-R. XviD and others provide excellent results, but for a beginner DivX is probably the safest bet.
If you go to Doom9
, there is a wealth of information regarding high quality transfer of DVD's to compressed files on CD-R.
I use the program called Gordian Knot for this purpose, and highly recommend it. It is available at Doom9 along with all of the freeware necessary to make high quality copies.