The rule of thumb about oiling your sewing machine is to put one drop of oil (sewing machine oil only, please!) wherever one metal part moves against another. Do that, and you won't go wrong. You can also look for the oil ports -- small holes in metal gears and housings--I've posted a good photo at www.robbiesews.blogspot.com--where you put one drop of sewing machine oil. Don't use oil on the gears themselves--use sewing machine grease instead.
Oil ports and places where metal moves against metal can be found in three locations: on the top of the machine, under the cover; on the bottom of the machine; and behind the access door (or panel) on the left side of the machine head.
Here's a generic sketch:
Note: Please refer to your sewing machine manual to see if your machine is self-oiling. If it is, disregard this note--you do not oil those machines.