I am an apparel technical designer that specailizes in fit. I have a couple of suggestions about what I will do when I get my sleeve and work hard and drop weight like crazy.
1) Fit is not generalize in the world we live in. I can wear a size 24 in store and a size 20 in another. Old Navy is not expensive and a good choice for trendy clothes. Start slow. If you bring jeans, blouse, dresses, and work wear into the dressing room you will be overwhelmed. If you are swimming in your pants, go to Old Navy and grab at least 3 sizes of jeans if you don't know your size. Try the biggest on first and work down from there. If skinny jeans don't look good but you found the right size, try a boot cut or straight or wide leg. Once you find, the right pair stop there and just buy the jeans. The next trip try shirt/blouses. Then the next dresses.
2) If you have a friend that knows how to sew take advantage of them. A good tailor/seamstress will know how to alter your existing clothing to fit you for a longer amount of time. A dress can go down a size with little problem. Pants will be more difficult. (I hate that it's winter and I can't wear a sun dress everyday.)
3)I suggest buying one staple piece like a black pants or a white blouse when you go down a size. You can mix and match with other clothing that may be a bit too big. Then when your white blouse is a size too big you could mix and match with a smaller size of jean or skirt.
4)If you do go into a store, ask the sales associates for help. Old Navy probably won't do this. If you tell them that you have had a sleeve, they will love playing dress up. especially if it is a slow day. Most sales associates will know what is on trend and you will be out of those cargo pants and in something you are comfortable with but still able to rock a trend that will only last as long as your current size.
5)Once you find a size, find a good thrift store. There are tons of great name brand stuff that is barely worn there. Consignment stores can be good too because you won't need those bigger sizes anymore.
6)Bras are going to be a hard one. Non padded bras that are stretchy and a bit smaller would be a good purchase or sports bras.
I hope this helps. I know it is frustrating when there are so many choice that you haven't had before. Take it one day at a time and enlist friends and family to help just like your weight loss.