Real estate agents charge a commission to help somebody find a home to buy. The seller is the one who pays the commission, which again depends on the price of the property.
If you want to rent an apartment you can use the service of a real estate agent but this is not very common. Your best bet is to do the research on your own.
If you use a real estate agent to find rental accommodation, you will have to pay a commission to the agent when you sign the contract. The cost of the commission depends on the rental time and can be anything between half a month and two months rent.
Some agents try to charge you an upfront ‘registration fee’ for using their service, which is sometimes called a deposit. We recommend you to never pay any money until you’ve actually found an apartment through an agent and you have signed the contract, or the agents might try to withhold the money you’ve spent – even if their services have proved completely useless.