I'm not expert on this topic, but it's an important topic that shouldn't go unanswered.

In my experience, writing tests and testable code needs quite some experience - it's not something you only learn from reading.

Some things to learn:

• Test framework
  • Assers
  • Mocking
  • Coverage
  • Reporting
  • Quality gates on commits (tests are only useful if they're run)
  • Are tests based on design/documentation?
• Conventions
  • How much coverage do you want/need? (lines or paths?)
  • Do you test private functions?
  • Do you write tests before (TDD) or after?
  • Ate tests written by the developer (more efficient but biased)?
• Which functionality is worth testing
  • Is it hard to test automatically?
  • Is it likely to break?
  • How serious is it if it breaks?
  • (Can it be a compile/lint error instead? Because that'd be preferable).
• Write testable functions
  • Allow mocking of prerequisites (by loose coupling with dependencies)
  • ...but don't go overboard with the dependency injection - you can't pass every dependency all the way from the main function and not everything needs an abstract factory pattern
  • Prevent side effects - make required state and expected results explicit (I prefer arguments and return values over changing state somewhere).
  • Try to isolate things like database access so it's easier to mock.

Writing testable code is really hard. Especially if there is a lot of database access or api calling going on.

The upside is that many qualities that make code testable also make it maintainable in general, especially loose coupling and small functions with a single responsibility.

I'd say start with Selenium if you plan on making use of the web based application testing.

It works with most of the languages. Namely Python, Java, C#, Javascript (node), etc.

Getting started is very easy. Though you'd need to make use of browser drivers like Firefox marionette gecko driver and chrome driver to start with. Follow Selenium 3 with Java and Firefox if you are into Java programming or Android. For python, you may have to install the PIP package for selenium and also have to configure the Firefox browser to run Selenium 3 with Python .

Now this is specifically in context of web based testing.

In case of Android, you may want to make use of the tools that test the UI, Network and other functionality differently. Single testing tool won't be able to do justice in such case.

Also do take a look at new tool named - Katalon Studio. I have just started with it and so far good experience. Again for web specific automation.

As you are a beginner, you must have to go through the following books to get deep into automation testing.

1. Clean Code: A Handbook of Agile Software Craftsmanship by Robert C. Martin <br>

2. The Just Enough Software Test Automation by Daniel J. Mosley and Bruce A. Posey <br>

3. Implementing Automated Software Testing: How to Save Time and Lower Costs While Raising Quality by Elfriede Dustin, Thom Garrett, Bernie Gauf <br>

4. Software Test Automation: Effective Use of Test Execution Tools by Mark Fewster and Dorothy Graham <br> Original Source: Top 10 Books for Getting Started with Automation Testing