I buy the biggest size when shopping :-(

when I was out shopping with my friends yesterday there were suggestin clothes for me and stuff because I couldnt decide what to get but they kept picking up clothes to small for me and stuff, you know like ones that fitted them. I then reliased I wear the biggest clothes within my friend group and my self esteem crumbled. I now feel like a piece of fat sh*t. I mean before I was convincing myself that I was ok and thin was hard to get and boring, but now I feel so crap, I wear a size 8 or 10, they wear size 6 or at the most 8(altohugh they complain often that its to big), sometimes even 4! I thought I was bought the same size as them, obviously I am a lot fatter than I thought! then I felt even worse when another friend said how annoying she couldnt find sizes small enough in the shop to fit her and she didnt know how some of the group found clothes to fit, but she didnt mention me so I reliased that I am seeing myself wrong, like smaller then I actually am. :-( so I have decided to loose some weight and go down a size or two.

so to my actual question, how much weight do you need to loose to go down a size? and any suggestions to how to do this and keep it off, forever!?!?!?!? Thanx x