Purchase a good quality oven thermometer from a kitchen supply store. Place it in the center of the oven and preheat the oven. This will tell if the oven's temperature scale is accurate, and if not, you now know how to adjust recipe settings as needed. For example, if your oven is 10°C warmer than the temperature shown on the dial, you should preheat the oven 10°C lower each time you use it.
You can also use a thermometer to check for hot spots in the oven, which can sometimes cause baked products to rise and brown unevenly. Yes, this is most likely why your cakes are always uneven and your cookies are never an even color! Each time you preheat the oven (to the same temperature) move the thermometer to a different spot and see if the temperature changes. This way you can see if your oven has hot spots. You can fix it or make sure you turn your pan or sheet at least once during baking to aid in cooking and browning.