Try a 36E (did you hold the tape measure tight?) and sizes around that. Make sure you get the back right first.
Measuring
Measure under your bust (hold the tape tight) to get your band measurement. Do not add on any inches to this measurement. Then measure across the fullest part of your bust. Each 1" of difference between the two measurements equals a cup size. So, let?s say you measure 30" underneath and 35" across? That would give you a starting point of 30DD.
Usually you should take the across bust measurement whilst wearing your best fitting bra (preferably an unpadded one although slight padding shouldn?t make too much of a difference). If you think that your bras are fitting so badly that you'll get a distorted measurement, then measure without a bra, but lean forward so that your boobs are hanging towards the floor (attractive!). The cup size is always harder to get right by measuring than the band size.
Testing the band and cup
To test the band size, put the bra (in this example a 30 back) on back to front on the loosest hooks. If you can breathe but if feels tight and you can only fit two fingers underneath the wire at the front, then that's the correct back size for you. If it's so tight it's unbearable then go up a size.
The cup will differ much more between brands and styles than the back. Turn the bra round the correct way, lean forward then take your right hand and put it inside your left cup. Reach right round under your armpit/towards your back and scoop all the backfat/escapage forward in to the cup of the bra. Repeat at the other side.
Then put the shoulder straps up and adjust as necessary (not too tight as it's the back that should be doing most of the supporting work). If for example, you've started with a 30D, you will probably find that after scooping, you have some bulging over the cup and four-boobage going on! This means you need to go up one or more cup sizes. Keep going up through the cup sizes until all your breast tissue is firmly in place.