Please. Being polite in any language will get you a lot farther than you think. It is always important to learn how to say please, that way even as you muddle your way through asking for something, at least you were polite!
Good morning/afternoon/evening. This way you can greet people! When you leave your hotel, when you enter a restaurant, or even passing someone on the street, it's nice to be able to offer a friendly greeting, in their language!
Where is … This beginning to a sentence is essential when it comes to navigating through a foreign country. You won't get very far if you don't know how to ask people where the train station is, that statue you've been searching for, etc. Knowing thing will be a huge help in making transportation and directions go more smoothly!
How much… For obvious reasons! Being able to ask how much something costs is important. Buying food at a market, you want to know how much to pay! Or when you hop into a taxi, asking how much it will cost you ahead of time can save you from getting ripped off when you get to your destination!
With these 5 phrases, you would be surprised how many situations you can get through! Of course, try and learn as much as you can, but start here. When people see you making an effort to speak their language (even if you're completely butchering it), they are more apt to want to help you. They are going to be more open to you, and you will find it makes interactions a lot easier!