The most common phrases I hear waiters/waitresses use are "what can I get you" and "what will you have"

"May I help you" and "what do you want to eat" also make sense and would work fine :)

EDIT: I also totally agree with Rosco, "Are you ready to order?" is also super common - often the conversation will go something like:

waiter/waitress: "Are you ready to order?"
me: (looking around to table to make sure noone disagrees) "I think so, yeah"

waiter/waitress: "great, what will you have?"

If I'm by myself, I'd probably respond with:
"I think so, yeah - I'll have..."

may I help you ? what you want to eat? "