1. Passage of One Lunar Year:
Zakat is obligatory after a time span of one lunar year passes with the money in the control of it's owner. Then the owner needs to pay 2.5% (or 1/40) of the money as Zakat. (A lunar year is approximately 355 days).
2. Deduction of Debts:
The owner should deduct any amount of money he or she borrowed from others; then check if the rest reaches the necessary nisab, then pays Zakat for it.