There's so much to it that it's impossible to help much in a forum post. Read, read, read about conversion rate optimization. Watch what big players like Amazon, Crate and Barrel, and Zappos do and try to implement things you see them doing.
Here's a couple of important factors (by no means all of them):
1) Phone number in the header so it's visible on every page.
2) Use the words 'Secure Checkout' on your site.
3) Have contact information, security logos visible on the cart page and through checkout process
4) Tell shipping prices up front and before they even put the item in cart if possible. Unknown shipping charges or charges that get added at the end of the checkout process kill conversions.
In general think about every objection your customer may have to purchasing from you, and every question they may have, and alleviate those objections and answer those questions early in the process.