Good question. Fortunately, you won't need to pay the entire purchase price out of pocket -- but you will need to come up with some serious cash early on in the home buying process.
First, you'll probably be expected to include a modest "earnest money deposit" with your offer, to show that you're serious. (You forfeit the money if the seller accepts your offer and you back out for no good reason -- or at least, no reason that's recognized under your contract.)
Next, plan on making a 20% down payment. (Before the burst of the real estate bubble, lower down payments were commonly accepted by lenders, but no more -- although you can still get a loan with less money down, you'll have to pay more interest for it. You'll also face extra scrutiny from the sellers, who will worry that you won't be able to close the deal with so little cash.)
You'll also need to pay closing costs -- the various fees, mortgage points, and other upfront payments associated with finalizing the purchase. These alone can run into the thousands of dollars.