“Actually, can I get plastic instead?”

You're checking out at a supermarket. The cashier starts putting your groceries in a paper bag. You'd rather have your groceries in a plastic bag instead. You quickly say:

Actually, can I get plastic instead?

Actually,

Use "Actually," to point out something that's unexpected or different than what the listener expects to hear:

Actually it's been kind of slow this week.

Actually, now's not a good time.

"Actually," is really useful in situations where someone is doing something wrong and you want to correct them. For example, if you're putting a puzzle together with someone and they put a piece in the wrong place, say:

Actually, the blue piece goes over here.

In the example at top, the cashier is putting the groceries into a kind of bag that the customer doesn't want, so the customer starts with "actually" to let her know that there's a problem.

a plastic bag

Most grocery stores in the U.S. offer you a choice between paper bags and plastic bags. Cashiers will often ask the customer:

Paper or plastic?

When you're in the checkout line at a grocery store, "paper" and "plastic" usually refer to paper and plastic bags.

can I get (something)

This is one good way to ask for something from a cashier, waiter, hotel clerk, flight attendant, etc.:

Can I get a glass of water?

Can I get an extra pillow?

Can I get change for a twenty?