Short answer. It means
You want this for here or to go?
So, this is always said when you buy food in shops, but not shops like supermarkets, but shops where you can actually eat the food inside (if you want).
So you've got two options:
- Taking your food to a table and eat it there.
- Having the food packed so that you can take it away to eat it later, wherever you want.
So the salesman is basically asking wether you are going to eat the food there or you want to take it away (having it packed, "to go").
The same applies for drinks.
The usual sentence is
¿ (Es) para tomar aquí o para llevar?
(Is it) for here or to go?
Because the salesman needs to know if (s)he has to pack it for you and put it in a bag, or just serve you the food directly.
So the answer is easy: it is in the very question.
Para tomar aquí = for here.
Para llevar = to go.
You can always add "por favor" at the end, that's more polite, but that's pretty much it. It's quite simple.