How to escape a MITM attack?

With the subject from above I have spent quite some time with Google Search. But there seems to be lacking some document that describes for a home user of an intermediate skill level how to escape an attacker who seems to sit between modem/router and the internet. The main trouble is that I have no way of proving the existence of the attacker. I dont want to go through the process again where I describe things that I experienced or discovered and people laugh about me or worse. Things happened like the popular lastpass app where the old passphrase was activated and the passwords inside the bin were deleted. My paypal account was added for Google Play. Things on the Deskop are pure trouble on solid distros like Debian or openSUSE).I have no clue how to proceed. Any tips are welcome.