I really can't offer much more insight than what has been said here already.
#1 - You need a domain. If you already have one, great. If not, you need to register one. Namecheap and Godaddy come to mind, for both price, service and features.
#2 - You need hosting. If you have your own server, great! Unless you're talking about some cheese machine running on your cable or DSL account which is probably against your provider's TOS. Cheap hosting is easy to come by. As have been said already - RossMAN, DeviousTrap, and many others can help you with this.
You don't need a lot. You certainly don't need your own "server". Shared hosting is just fine. A handful of meg of storage to have your product descriptions and images as well as your item database is all you need. Bandwidth - even 1GB/mo should suffice unless you plan on turning over hundreds-of-thousands of dollars a month. You're talking free to single-digit dollars per month.
You will also need on your hosting package database access and scripting/CGI support. Most common, as mentioned already is PHP and MySQL. You will need this for running any sort of eCommerce site. There are other options, but most use PHP and MySQL. This should come with your hosting package.
#3 - You need to choose an eCommerce package. osCommerce, which has been mentioned numerous times already, is your most likely candidate. osCommerce is easy to use, and a nice templateable front end. You can customize it any way you like.
#4 - If you want people to trust you, an SSL certificate. It basically secures data going between your and ensures to the customer that they aren't actually at a 3rd party scam site. Some servers offer shared SSL certs at a discount rate, otherwise expect to pay a hefty pricetag for your very own cert.
#5 - Payment options. Obviously paypal is in order for online transactions. However, a lot of people don't want to open up an account with paypal in order to take credit cards. In this case, you'll have to research merchant accounts and tools for accepting credit cards. Expect to pay a fee play a percentage of each transaction for this. Again, unless you're going to turn over a lot of money every month, this alone can run a small site into the ground.
Once you've got this ironed out, then you put your site together and advertise. That's another lesson in itself. The above 5 things should be your first concerns.
edit - I should add, one very, VERY important thing to consider is how you are going to set up your supply and shipment chain too. It is very important that if you're a small time site op you collect your orders as frequently as possible, and ensure the orders are filled promptly and accurately. You'll have to manually take care of inventory in most cases, ensuring that your site is up to date on in-stock and out-of-stock items, and stock on hand is accurate as well. No customer will return to you if they continually see that you have something listed as in-stock and place an order only to get an email saying "Sorry, that item is currently out of stock..." While this isn't really eCommerce related, this too is something that should be seriously considered in your first look at setting up.
Facebook
Twitter