Micronics Group Study

I just bought my first car last month, a little used hatchback that I saved up for forever. I took it in for a basic checkup because I wanted to make sure everything was okay and the mechanic quoted me almost four hundred dollars for stuff I barely understood. When I looked at the estimate later, I saw they wanted over a hundred bucks just for an air filter that I could probably buy for twenty dollars. It really opened my eyes to how important it is to understand what you are paying for. I have been researching ways of minimizing car workshop charges because I cannot afford to get ripped off every time I need an oil change. I learned that you can ask for the old parts back and that sometimes shops add services you never asked for. Does anyone have advice on how to tell a trustworthy mechanic from one that just sees a young person and thinks easy money?