I would like to share a couple of framworks that are very useful for testing Apex Code.
The first one is Apex Mock and it will become essential for you when it comes to create test data for your Apex Test classes.
The second framework is FinancialForce ApexMocks Framework. This one allows you to decouple complex logic and test it separately.
I encourage you to check them out and spend half an hour trying to understand how they work. Once you have done that, you will save lots of hours creating your tests.