Two burgers to take away, please.

[Xinfu]

If we want to get the food and eat else where we might say

-Two burgers to take away, please. (I guess in the US away will be out)

But how about if I want to 'canteen/restaurant/shop eat'? What should I tell the staff members?

[Rusty]

If we want to get (the) food and eat (it) elsewhere, we might say

-Two burgers to take away, please. (I guess in the US away would be out.)

But how about if I want to eat it at the restaurant? What should I tell the staff members?Two burgers to go.
Two burgers for takeout.
Two burgers for carryout.
Two ... to take-away/takeaway. (BrE)

Most staff will ask, "Is that for here or to go?"
To go.
For here.

When you arrive at a restaurant or eatery, you are either seated and order (if you're staying to eat) or you order at a counter (if you're getting takeout/the food to go). Then there's no question. If you decide you don't have time to stay and eat, you can ask, "Can we get that to go, please?"

If you call out for pizza, you're asked, "For pickup or delivery?" If you want to drive to the store to get your order, you reply "For pickup", or "I'll come and get it." If you want someone to deliver your order to your house, you say "It's for delivery."

[Xinfu]

Thank you, Rusty~