How do you get DSi points?

If you are reffering to the points used in the DSi shop, then they must be purchased.

From the DSi shop, click 'Start Shopping', then 'Add Nintendo DSi Points'. You have a choice then of purchasing using a credit card, or redeeming a Nintendo Points Card. If you pay with a credit card they cost £9 for 1000, £18 for 2000, or £27 for 3000.

You are also given an introductory 1000 points for free when you first use the DSi shop.