Average speed depends a lot on various factors. But, having said that, 8km/h is very slow for a fit young person: it's not a whole lot more than walking pace.
Where you're cycling. In cities, it can be hard to cycle quickly. Cycle paths often have lots of pedestrians on them; roads often have lots of junctions, stop signs and traffic lights. If you're constantly having to stop and get going again, that will bring your average speed down a lot.
You say you're in a hilly city. Going up-hill is, of course, slower. However, on average, you'll go down as many hills as you go up, so that should balance out to some extent. If you mean that you average 8km/h up the hills, that might not be at all bad, depending on how steep the hills are!
How you measure speed. If you just compute distance travelled divided by time taken, then you're including all the time you're stopped in your average: if you cycle at 15km/h for ten minutes and spend five minutes stopped, your overall average would be 10km/h but, when you were actually cyling, you were doing 15. If you're using a cycle computer or GPS app, you should set it up to only compute average speed over the time you're actually moving.
How you cycle. Obviously, if you don't try to cycle fast, you won't be fast. Are you working quite hard when you cycle, or are you just drifting along? Do you get out of breath? Are you using your gears properly? A lot of beginner cyclists turn the pedals very slowly in much too difficult a gear, and it's hard to go fast that way (and very hard to accelerate away from a stop). As a rough guide, if you're pedalling at less than 60rpm on flat ground (i.e., pressing your right leg down once every second), you're pedalling very slowly and you should probably aim for something more like 75. Use an easier gear to compensate and, crucially, if you feel yourself slowing down, change into an easier gear. The point of the gears is that they let you keep the pedals spinning at a good rate whatever speed you're actually going at. A good way to measure how fast you're pedalling is to use the internet to find out how many beats per minute there are in some songs you know, and compare that against how often you're pressing down your right leg.
How well your bike is set up and maintained. Many beginners have their saddle much too low. An easy rule of thumb is that, when you're sat on the saddle with your foot on the pedal at its lowest point, that leg should be almost straight. If your saddle is much lower than that, it's really hard to generate any power. Is your bike well maintained? Do the wheels spin freely? Are the brakes rubbing? Is the chain properly lubricated? Does the bike make noises while you cycle?
