Tax deduction: who can get it and how

I am officially employed and pay taxes from my salary every year. I've seen that it's possible to get a part of the taxes back through a tax deduction if there are confirmed expenses, for example, for education or medical treatment. However, it's not entirely clear what specific list of expenses qualifies and what documents I need to collect to guarantee I receive such a deduction. Please advise who is eligible for a tax deduction and for what expenses it can be applied?