The Better Bra Campaign website is here:
thebetterbracampaign.blogspot.co.uk/p/fitting-advice.html
You need to measure under your bust, pulling the tape quite firmly - that is the bra band measurement (none of this "add 4 inches" nonsense that M&S etc still do). If you're between sizes, then probably size down. Then measure around your boobs while leaning forward. This might sound awful (or did to me at least!) but go with it!
Then for every inch that measures more than your underbust measurement, go up a cup size - it starts at an A cup for 1" more and goes: A,B,C,D,DD,E,F,FF,G,GG,H,HH,J,JJ,K,KK,L... The bra size you get will then be your starting point for trying a load of new bras. Bravissimo may be your best bet. You may need to try a few different styles and a few sizes around your starting point to get your perfect bra.
If you post your measurements, then the MN bra gurus will help you! Or you can work it out yourself if you prefer.
Also don't forget to "scoop" as described in the link - it makes a massive difference.
It might seem like a hassle - but, really, it's so worth it.