If you immigrated to the Netherlands in this calendar year, this year is your migration year. You file it with the M-form (M-aangifte): only your Dutch income up to your arrival date, and your worldwide income as a domestic taxpayer from that date. Only from the first full calendar year onward do you file the ordinary income tax return. Taxmo files the M-form directly to the Belastingdienst, from €129 incl. VAT.
From your arrival date you are a domestic taxpayer and your worldwide income counts, including a Dutch salary with the 30% ruling if you have one. Before that date only Dutch-source income counts, for example a trial period or a first pay period that started before your actual move.
For the box 3 savings-and-investments basis, only the period you were a domestic taxpayer counts. Savings or investments you already held abroad before moving fall outside Dutch box 3 until your immigration date. The valuation date for the calculation stays 1 January, even if you arrived later in the year.
The screening questions tell you straight away whether your immigration year can be filed online. From there you go through your Dutch and foreign income, a fiscal partner if you have one, an own home, and box 2 or box 3 where they apply. If you had the 30% ruling, it is already reflected on your annual statement. You only pay once the return is ready to file.