Basic measurements are (while wearing a decent fitting bra):
Around the top of the chest/under armpits
Around the fullest part of the breast (where the nipples are)
And then around the rib cage just under the breast.
Make sure your tape measure is not on any breast tissue with the bottom measurement. I've had a lot of customers come in and tell me that they were last measured at XX when, in fact, I just measured them at XX-3 because the last person to measure them didn't make sure that breast tissue and/or bra underwires weren't included in the measurement. I've even gone so far to ask women to please lift their breasts up so that I could ensure I wasn't measuring the bottom of their breast tissue. (Once the tape measure was in place, I had them let go)
Your band size will fall between the measurement under the armpits and the one around the ribcage. I can't give you an exact # because the rest of your structure will dictate which it is but it does give you a ball park figure.
For example, if I measure a customer at 38" under her armpits; 40" around her fullest part; and 36" around her ribcage, I will suggest a 38B and a 36D as places to start. You won't know what fits until the bra is on. From then, change only the size to find the correct one in that bra. Don't change bra and size unless it's super obvious that both are not suited.
If all your bras have padding in them, and that's what you're wearing when you measure, subtract a 1/2" or so or just measure a little snugger...not tight, but snug.
With fit, you want to ensure there are only two breasts. You don't want two hanging out the center and you don't want two squishing into your armpits. If you have this problem, try a larger cup.
If your breasts are falling out the bottom of the bra, the band is too big/stretched out.
If the band keeps hiking up in back, go down a band size but get the comparable cup size. It will be the same cup coverage but snugger in the band.
You want the center gore (the part between the cups) to be flat/flush against your skin. You should not be able to put you keys in this space--it's not a pocket. If the center gore isn't flat, go up a cup and down a band--see below for conversion.
Don't be afraid of letters beyond DD. It is what it is. Wouldn't you rather have a bra that fits rather than a specific size. Though, I did encounter a customer who refused to believe she was a 36DDD and only because she had to have a VS bra.
32C=34B or 30D
34D=36C or 32DD
36DD=38D or 34DDD
38DDD=40DD or 36DDDD
D=D (4" difference between band size and cup size)
One last tidbit...we all have mismatched breasts...some as much as a cup difference. Fit the larger cup. You can always add to the smaller one. You can't make the larger one smaller with a bra.
Can't find your size in stores, try figleaves.com, barenecessities.com, or herroom.com. They all have beautiful bras in a huge range of sizes...all the way from 28A to 52J.