Convenience stores in Japan provide a variety of services, from printing to currency exchange. While the airport is often English-friendly, things might be different once you step outside into the "real world".
Here are some useful phrases you might want to remember for your first few days in Japan.

Services:
この札をくずしていただけませんか。
↬ Can you break this bill (into change)?
For example, you can say:
↬ 1000円札（えんさつ）をくずしていただけませんか (Can I have change for a 1000 yen bill?)

If you need to pay a bill with a QR code at the register:
これを払うことができますか。
↬ Can I pay this?

If you need a specific item:
Xはありますか。
↬ Do you have X?

If you need the clerk to heat up your bento:
温めてください。
↬ Can you heat this up?

Common Responses:
The clerk will usually ask if you want a bag:
↬「レジ袋いりますか」 or「レジ袋どうなさいますか」＝ Do you need a bag?
↬「このままでいいですか」＝ Do you need a bag? (Literally, "is this okay as it is").
In response, you can say:
大丈夫です。
↬ No, that's okay. (You can wave "no" while saying this to make it clear.)
はい、お願いします。
↬ Yes, please. Thank you.

The clerk will also ask if you want a receipt:
↬ レシートは宜しいですか？ ＝ Do you need a receipt?
Again, you can reply:
いえ、大丈夫です。
↬ No, that's okay.
はい、お願いします。
↬ Yes, please. Thank you.

If there are any more useful phrases that you know, feel free to post them below.