When you click the sell button it will walk you through that part of the process for the most part. Do you have a paypal account? If not I would get one because people who don't use it don't make nearly as much because nearly everyone on eBay uses it as the preferred payment method. You could go a few routes with shipping clothing. If it falls under at 13 ounces you can ship first class and that's rarely over $3.50, but most likely it comes in at over 13 ounces. When that's the case you have the options of shipping parcel post which is the slowest method but generally cheapest or priority that gets it there in 3 days or less anywhere in the US. You can request FREE boxes and envelopes on the USPS website for priority shipping, but not any other shipping methods. They have new padded flat rate envelopes that would probably fit most clothing items as long as they aren't too thick that cost around $5 to ship. It's slightly less if you print your shipping label directly from ebay or paypal and you get delivery confirmation if you ship that way. If you take it to the post office delivery confirmation is 75 cents more.
I could probably answer most any specific question you have about eBay or Paypal, but I'm not very great at laying out the entire process at once in text. If you know specific weights I could probably help more with shipping options.
Oh and the best advice you can get for selling on eBay... Make sure your pictures are great. Nothing blurry or dark and don't have much clutter in the back ground. The better your pictures are the better your sales will be.