It depends on your situation. If you want to get a conventional loan and have good credit, you can put as little as 3% of the purchase price down. But if you don't have enough savings and can't afford private mortgage insurance, it's better to have at least 20% equity in your home when buying it.
If you're looking for an FHA loan, most common among first-time buyers, then 3.5% of the purchase price will cover things with a credit score of 580+, while 10% will cover things with a credit score of 500-579. If your credit score isn't quite so great, then some lenders may require that you put down as much as 25% of the purchase price.