You can find your IBAN through official channels provided by your bank: mobile app, internet banking, account statement, or branch.
Steps
Some banks also offer an IBAN tool on their website.
Don't try to create your IBAN yourself from your account number, as even a correct account number doesn't guarantee the formatting is accurate.
Before sending it to anyone, copy it from your bank's official website and double-check it, especially for international transfers.