The internet is easy to navigate, though it does take some getting used to if you're new to it, but that doesn't mean that a successful online business is as easy as creating a free eBay account and calling it good. You have to put a little knuckle grease (because let's face it, typing doesn't really use your elbows much) into being successful at any small business, even one where you are a faceless seller behind a strange user name.
To accurately measure how much shipping will cost for each item, you'll need a scale. You can use a cheap kitchen scale if you generally sell only small items, but if you get anything larger, it'll be a huge trial to try and balance them on the tiny surface of a scale made to measure apples. You can buy a postal scale made to weight packages for as low as $20 -- and if this is a legitimate business venture, that's money that may very well be worth it!
These days, with credit card fraud so easy to perpetuate, a Paypal account is a necessity. You can sign up for an account for free, and attach either your own credit card or a bank account. Paypal is helpful both for buyers and sellers -- as a buyer, you don't have to worry about all the different types of payment you do and don't accept, and can just offer Paypal as an option. Your customers will feel more secure knowing that you can't use their credit card information to defraud them.
Unless you're only doing local business, you'll need to acquire a large amount of packing materials. This can mean anything from envelopes and packing tape to send off fabric patches and customized bookmarks, to giant moving boxes to send items you bought off craigslist for resale. You'll also want some sort of cushioning material in bulk; balled-up newspaper works well, as do packing peanuts, and depending on the insurance policies of the company you ship with, you may need to use a specific type of packing materials.
If your business is really serious, and you don't want to be making regular trips to the post office, you may consider investing in a postage printer. These devices are licensed by the post office, and once you pay them for the cost of shipping, the label spits out right in your own home. Make sure your post person can pick up the packages for you!
Get some materials to sell online at garage sales! It's a great place to make discoveries and make a little money off those great finds.