What maintenance you do and how often depends on how much you are riding. The following table assumes daily riding; if you ride less then adjust accordingly. Check the following:

If you pay attention as you ride you will be able to notice any changes in your bike, for instance if the gears no longer change smoothly or the handling feels a little odd. Bike maintenance is best done 'a little and often'. As you learn how to spot problems, you will know what to fix and when.