Check your owners manual, or if you can find one, a service manual. It has lots of useful information about your bike, including a maintenance schedule. For oil it says every 8,000km or every 12 months, whichever comes first. Tribology is a very argumentative subject, and there are a lot of people who will tell you the way you should be doing it. A lot of this is more opinion than fact. To a certain extent you have to decide for yourself how often you want to change it. Over time oil will break down and loose its lubricating properties. It will also absorb contaminates from the combustion process, eventually becoming saturated. I would never go beyond what the manual suggests, but changing it too frequently is a waste of money and resources. The best advice I can give is to do some research.
Again, check the manual. There are a number of things that need to be serviced by someone capable on a schedule. There are also some items that you should check weekly, or even daily, before you ride:
a. Oil level. Running low on oil is the quickest way to destroy a bike short of riding it off a cliff. The level shouldn't ever go up, but it will often go down if the bike burns a small amount. Check the owners manual for the proper procedure when checking. Most bikes need to be level, and not on their sidestand.
b. The chain. You should clean and lube it regularly. A good rule of thumb is every 2-3 tanks of gas, or after every ride in the rain. Also keep an eye on the amount of slack it has at different points.
c. Tires. They need to be inflated properly. They can lose pressure over time, and their relative pressure changes with ambient temperature. Make sure you check them when they're cold, before you ride. Their temperature and pressure will go up when you ride. Check the treads and sidewalls for cracks, damage, or foreign objects. Check the wear indicators or measure the available tread depth using a gauge. Ideally you don't want to ride on tires more than about five years old, regardless of how much tread they have.
d. Brakes. Make sure the pads have lots of meat left on them. Make sure the fluid level is between the lines and there are no leaks anywhere.
e. Forks. Check the seals by looking for oil leaking where the inner and outer tubes meet. If it does leak, it will likely end up down by your front brakes, so it's dangerous if you don't notice.
f. Misc hardware. Check any bolts you can to make sure they aren't loose. You shouldn't see threads showing, and you shouldn't be able to spin them with your bare hands.
g. Coolant. Make sure it's between the lines on the reservoir.