My suggestion is to read the following items:
Web Testing 101 | MoT (You need a pro account to access all modules.)
Cross Browser Testing: Getting Started
Getting started with mobile testing?
Accessibility Testing Resources Needed
I wrote a blog post serie about flaky tests in a web application. Tests have different test results within minutes, while not a single line of code in the test code and the application code has been changed. This struggle took me several months to solve it.
Another interesting thing to explore is to use TDD in Test Automation: