The mortgage will dictate the downpayment. You can write the offer correctly to minimize your out of pocket costs.
To get a good loan, expect 3.5%-5% down. You can often get a lower rate with a larger downpayment. If you use some of the grant programs, you can get a house with less down, but you will pay for it in the long run.
Matt Helpling at Rapid Mortgage is a good resource for you.