It’s simple you pay tax when you begin to earn income of more than the legislated amount for that year and you pay it all your working life. This tax is worked out on a sliding scale, see this scales in the tax tables below.
Tax works in tax years. The year starts 1st March and ends on the last day of February. You need to submit a return in Filing Season.
Watch the following video to gain more insight into the various types of taxation: