I would say...
"yes, how can I help you?"
"sure, go right ahead."
or
just "Yes' is good way to answer.
something like that...
about "shoot' i think it's really depends on how they talk.
If i ask somebody, and he whispered "shoot..." it's probably means he doesn't want to hear me or answer.
people use "shoot" as a "sh*t" a lot as i feel.
but of course, "shoot' means also, as he said "go ahead".