Hi there, from a seller's point of view, I would say that a down payment is absolutely essential to secure a deal. You've gotten good responses that show that there are many answers depending on financing alternatives and your financial position. But remember that the seller will evaluate each component of your offer, and one way to evaluate strength and commitment is by the size of the down payment.
I would also add a word of caution - as a newcomer to the process, be cautious about buying a home with little to nothing down. When you do that, the slightest downward swing in value essentially puts you under water. That is largely why we are where we are now with short sales and foreclosures.
Find yourself a great agent that has experience - and enjoys working with first time buyers. She/he can explain the entire process and should be with you every step of the day. She/he can also refer you to good mortgage resources. The better the resources that support you, the smoother your transaction will be. Good luck!!
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