In most cases you will be offered a tenancy agreement. Did you not get a tenancy agreement? Then keep all receipts or bank statements relating to payment of the rent. These at least are proof that you have an oral tenancy agreement. If you are paying the rent in cash, ask for a receipt, or create one yourself for the landlord to sign. Make sure that your name, address and the month are mentioned.
If you are paying your rent via the bank, mention "Rent, address + month".