A simple black dress can go formal if you pick nice fabrics like velvet or silk
Beyond that: choose fabrics that skim rather than cling, look for necklines (collars, V-necks, cowl necks) that frame your face, and select a length that flatters your proportions midi for most women, slightly shorter for petite frames
Choose one that is high quality it should be very smooth , creamy (with no chunks) with a drippy texture
But, if dramatic fashion isn't for you, the gown can also be ordered with simpler flutter sleeves and no cape
See Sarah and Lukes English Countryside Wedding Dark Navy Floor-Length Gown Several dark navy floor-length gowns one-shoulder, halter, and straight necklines all in the same rich color