Artemis, I'm sorry if I'm repeating what others have said, but this was a huge issue for me as I lost so I want to reply but don't have time to read all the other posts.
I found that it took a lot of lost pounds to drop a dress size in the beginning. In the US, 16 is the top size for regular women (and it used to be 14 and still is in some stores). I was a size 28.
It took me at least 40 pounds (maybe 50?) to get down to a 26, and another 40-50 to get down to 24. I was at about 20 I think, when I'd lost a little over 100 pounds. Since then, ever 10-15 pounds drops another size. I'm in size 16 pants, but they're loose, and into size 12 skirts, and I've lost about 125 pounds or so.
It's incredibly confusing. But keep in mind that there is a bigger gap between the larger sizes than the smaller ones.
I spent most of my time combing second hand stores for my clothes and I used clothespins on many of my old things. It really helped keep the cost down! I'm selling my old things at consignment stores. Seemed cheaper and less hassle than e-bay, but that's also an option.
Hope that helps!