If you are A-Shaped; narrow on top and wide on the bottom, then you should:
Opt for: a strapless ball gown that will cover your bottom half and put the focus on your better half. Also, an empire dress will hug the tiniest part of your body and disguise your lower half.
Say no to: a sheath that will most probably be unflattering to your body type. A trumpet or mermaid-style gown will draw attention to the bottom half of your body.
For more details on the perfect wedding dress for each shape, please check: http://www.arabiaweddings.com/tips/fashion/perfect-dress-perfect-fit
Please let us know if you need any help or have any further concerns.