Normally the rent has to be paid by the 15th. Your first bill might be a bit late, meaning if you move in on the 5th of January you might receive your first bill in February (you will then have to pay for 1 and ½ months).
As a tenant, it is your responsibility to obtain your invoice and ensure you pay it on time.
Log in to the Housing Portal to find your invoice. You may pay online or at the post office (Norwegian card or money only). All payments made from foreign accounts will have to pay bank fees.
Firstly to your own bank (different banks have different fees), secondly to Samskipnadens bank; if you pay in NOK the fee is 100 NOK, if you pay with EURO the bank fee is 20 NOK. If you wish to pay your rent via direct debit or electronic invoice, you can arrange this via your own Norwegian online bank.