Summarize:

Each sewing machine brand is different, so it's a good idea to check the manual that came with your sewing machine for directions on how to clean and oil the machine. Some manufacturers recommend cleaning the machine after every 10 hours you use it. Clean it when you see lint starting to gather. Some older machines mark the spots where you should drop oil in red. Others tell you where to drop it using pictures to guide you. If you don't have a copy of the instructional manual, you should be able to get one on the manufacturer's website. You might even be able to download it. If that doesn't work, call the manufacturer and ask for one. You will be asked the machine name, model, and serial number most likely. You could also ask a local dealer. Some machines do not need to be oiled. They are self-lubricating. Such a machine will still need to be maintained, but if it says not to oil it at home, don't.
Follow the instructional manual.