Your own maxim
Test a rule from your everyday life with Kant's questions — including the formula of humanity.
Persons exist as ends in themselves: they have absolute worth and may never be used merely as means.
The humanity formula of the categorical imperative puts it plainly: “Act so that you treat humanity, whether in your own person or in the person of any other, always at the same time as an end, never merely as a means.” Because human beings possess practical reason, they have an inner worth that cannot be traded off.
Involving others as means is permitted — we do it constantly, for instance when shopping. What is forbidden is treating them merely as means: deceiving, coercing or using them as if they were things with a price rather than persons with dignity.
Source: Wikipedia: End in itself