Translation:Nei, vær så snill!
As jpiperenger said, the default Google keyboard should have a globe symbol to the left of the spacebar. If you long press the globe and choose "language settings", you can add other language keyboards, like Norwegian. Press the globe to cycle thru your language keyboards to choose the one you want. But if you already have the default Google English keyboard, just long press "a" or "o" to find å, æ, or ø.
Takk = thanks/ thank you
Vær så snill = Please
Both are very common expressions, but here are a few situations where it would be natural to use them: "Takk" is often used when recieving something (a gift, a cup of coffe, a letter), when someone does you a favour.
"Vær så snill" can be used for asking for something, asking for a favour/help, pleading/begging.
However, both "takk" and "vær så snill" can be used when asking/ordering something: "Kan jeg få en kopp kaffe, takk/vær så snill?" (Can I have a cup of coffee please?"
If your intention is to order someone politely to do something, you can often replace 'vær så snill' med 'vennligst', which admittedly isn't that much shorter. Beware that it can come off as a bit formal, and also would not work in the above sentence.
It's something you'll often see written on signs, to soften what is actually an order:
"Vennligst lukk døren etter deg når du går."
"Please close the door behind you when you leave."
Quite the opposite actually, "Nei, er du snill" does work perfectly fine in some situations. For example: "Nei, er du snill å la døra være lukket" (No, please leave the door closed) "Er du snill" can be used instead of "vær så snill": "Er du snill/vær så snill å åpne døra" (Please open the door)