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.