I'd suggest you take someone with you when you go shopping and get them to pick a few outfits out for you. This is what I did after I had several unsuccessful and depressing shopping trips for a wedding earlier this year. My mum helped me and found a two piece outfit that showed off my best assets (boobs) and hid my worst (tummy).

You absolutely have to start trying things on and don't give up until you find something that looks great. If I can do it when I'm size 14-16 on the bottom half & size 16-18 on the top half I reckon you can easily do it at a size 12. Things can look completely different hanging on the rack so just grab colours and fabrics you like and just try them on. Good luck.