Earning by doing a part time job with studies?

Please note that, as a foreign student in Germany, you are only allowed to work a limited number of hours in jobs related to your studies (e.g. trainee or internship). It is not possible to earn your living expenses that way. Since studies are designed to be full-time, your results will also suffer if you do more than this.

But regarding your actual question: Shop clerk positions (and especially for those who do not speak fluent German, thus cannot face customers, but only re-stock shelves or similar) are very badly paid in Germany, due to many desperate job-seekers from Eastern or Southern Europe. You are lucky if you receive the (not yet universally enforced - and students are excempted from the rule anyway) minimum wage of €8.50/hour. Taxes and social security deductions will take about 30% of this.