If you're good at fixing bikes then swapping a new rear cassette should be fairly easy. It's possible that your gearing and/or hills are are such that you'll see an immediate benefit from that upgrade.
Otherwise, just ride more. Hills get everybody. Especially if you're new and your power output is low or you are overweight since gravity will have more effect on you. (Well, the same effect but there's more of you to have an effect on)
If you put out 100 Watts and have a combined rider/bike weight of 100kg then your speed will drop about 25% just from a 1 degree incline and 50% from a 2 degree slope. And that's not even very steep! It's a 60% and 68% drop at 3 and 4 degrees.
At the other end you might have a rider putting out 250 Watts and a cyclist/bike weight of 80kg. The speed drop here is only 12% for 1 degree and 23% for 2 degrees. It's about 30% and 40% for 3 and 4 degrees.
The 250 Watt/80kg rider is going about 50% faster than the 100 Watt/100kg rider on flat terrain but 110% faster at 2 degrees.
Tl;dr: hills hurt new cyclists more because their power/weight ratios are so much lower.