Q: How can I know my postcode?

Are you attempting to set up an account for a store other than the one for the country in which you live? If so, you can't.

If you are trying to set up an account in the store for the country in which you do live, you want to enter your billing address including whatever postal code is associated with that.

As far as I can see, there is no iTunes store facility available for Burma (Myanmar).

That means you cannot buy Apple store products there or anywhere else unless you have the required residential billing address and bank account / credit card.

iTunes Store: Which types of items can I buy in my country?