Here is a good guide on how to take your measurements for women's clothing. To get the right size, measure carefully, and update your measurements regularly.
When you have your correct measurements, you can easily find your US size.
To find your correct size, use a tape measure and check these measurements:
Measure under your armpits, around your shoulder blades, and over the fullest part of your bust. Don't pull the tape measure too hard. For fitting a bra, you also need the measurement just under your bust.
Measure around your natural waistline. This is the narrow part of your waist, approx. an inch above your navel. Relax and breath out before you measure.
The hip should be measured around its fullest part (about 8 inch. below your waist).
This is the measurement from your ankle to your groin. This can also be measured on a pair of trousers with the proper length.