Hi,
You should create an account move on the start of the fiscalyear with the opening credits and debits. In order to do so, you should go to the Financial → Entries → Account Moves and create a new record. Once you’ve selected a journal you will be able to define one line per account. On each line you should enter the opening credit/debit.
After entering all the lines you should click the Post button in order to confirm this move.