ELI5: How do I transition from proprietary software to free and open source software? Is there good free/open software on mobile/cloud?

I have recently read that non-open software is potentially harmful and can be a carrier for unwarranted data collection and can leave me open for possible attack. Is it even possible to use a mobile phone while using completely open programs? How do you replace useful tools that Google and "the cloud" offer. I have recently started to learn to program at school but I want to keep my computers safe from 3rd parties and harmful attackers that steal files and passwords. Please explain the best way to transition while giving up the least amount of comfort.