Vimshottari dasha divides a 120-year life into nine planetary periods: Ketu 7, Venus 20, Sun 6, Moon 10, Mars 7, Rahu 18, Jupiter 16, Saturn 19 and Mercury 17 years.
How your first dasha is decided
The nakshatra your Moon occupied at birth decides which planet's period you start in. How far the Moon had travelled through that nakshatra decides how much of that period was already used up — the remainder is your dasha balance at birth.
Antardasha
Each Mahadasha is subdivided into nine Antardashas in the same order, starting with its own lord. The results of a period depend on where the lord sits in your chart and which houses it rules.