During the year, keep all documents for tax records purposes to be able to calculate your expenses. Within your records of income you can also use flat-rate expenses and then you only need to keep receipts and issued invoices.
Once the year has passed, it is your obligation to submit a tax return. Tax returns usually have to be submitted for the previous year at the tax office of the place of your business by the end of March of the following year. If you have your tax return made by a tax consultant, the deadline for submission is extended until the end of June.
Detailed information on how to file a tax return and on tax rates together with other useful information can be found on the website administered by the Ministry of Industry and Trade:
A list of tax offices can be found here: