I know it can be kind of embarassing but the Lingerie people at Dillards (or Macys) are awesome and will "fit" you for free. They really do a great job of getting you the right size and helping you find a bra that best fits your needs. Here is how they told me to do it though....

Measure around your back, under you arms, across your chest...mine is a 30

Measure around your back and across your nipples... mine is a 32

SO..... 30 is my band size and then you count between the 2 numbers so 31 would equal an A and 32 equals B so I am a 30B. Maybe that will help a little. I really like Wacoal petite bras. They fit perfectly for my small bust and I can get them with lots of padding haha (you can also get them with little or no padding if you prefer though). Good Luck!