>>106938871 (OP)
If you want real answer this job is a lot like being a plumber or something like that. I don't know what are laws in your country, but in order to safely repair stuff for people you first need to register yourself officially. It will save you from people taking you to court saying you broke their garbage on purpose. Next you make website, business cards, posters, advertisement, etc. Your market is your local grandpas and grandmas and occasionally moms with broken samsung galaxy. Your city will probably already have similar businesses, so unless you know what you're doing I would suggest to first contact them and ask if they need any new hire.
If your question is how to learn to repair this shit, then it's mostly electrical engineering and experience. You can train by buying scrap metal like broken thinkpads and other tinkertranny garbage, repairing it and re-selling it as "professionally refurbished". Not gonna make a lot of money, if any, but at least it's real experience.