An apartment is held to be rented by you as soon as both you and the landlord have signed the lease. Your signature legally binds you to the terms of the contract. In particular, the lease covers the monthly rent and utility, the period of notice to be given for terminating the lease, payment for necessary repairs, responsibility for redecoration costs when moving out, duration of the contract and the terms for rent increase.
Furthermore, the lease may include additional agreements (e.g. use of the garden, car parking facilities etc.).
The landlord should be informed well in advance of plans to cancel or extend the contract period.