what kind of underwear
what kind of bra should i have with backless halter neck with sweatheart neck line?
Depends on how big your bust is, but you can buy stick on cups / bras (try figleaves) - if you want something more secure, I'm sure figleaves do other stuff too... good luck!
What about one of the multi-way bras available? Figleaves or the new M&S bridal may help.