The 20% is best, as it keeps you from paying PMI (Private Mortgage Insurance), which is insurance for the bank in case you default. PMI is money thrown away if you have 20% for the down.
If you don't, not to worry. I borrowed 103% on my first home, so I was really making a negative 3% down payment. That is, I got the home, and left closing with some cash to fix up the house, which made it more valuable.
You can do anything in between. If you go with FHA, they will require 3% down, and you have to show that you came up with it (not a gift from mom or dad). The exception is that there is a non-profit company that will 'give' you the 3% in exchange for a small fee, and the lender will roll that 3% into the loan, giving you, in essence, 100% of the home price.
I just sold a house to a young couple using this service. They otherwise could not have bought the house, and would have been stuck paying $925 per month for a 2bd apartment. Now they pay $750 for a 1,400 sq/ft house per month, and that amount includes PMI, taxes, and insurance.
Whatever you do, do NOT put down MORE than 20%, as interest rates are so low. If you have more money, keep it in investments that will make more than you pay in interest on your home loan, or use it to pay of credit cards if you have them.