These are some useful words or phrases you might hear or need to use in Japan. I hope this helps. I'm going to add more words when I think of them.
Namae wa nan desu ka? (名前は何ですか) or the less formal namae wa? (名前は) What is your name?
Genki? (元気) to make this a question you need to rise the intonation (gen) low (ki) high. It means "are you well/energetic?"
Otsukare (お疲れ) It literally means you are tired but it is a common way of saying goodbye when you are leaving. Even if you are not tired, it just a way of excusing yourself from the group to leave.
Mata ne (またね) or Ja ne (じゃね) NEVER SAY SAYONARA in a normal setting. Contrary to popular belief, the contation of the word is like the 'final goodbye.' You only say this to people to are not going to see for a very long time. Saying mata ne or ja ne means see you later.
Omattase お待たせ) "sorry for keeping you" or "sorry for the wait." The politer version is omattese shimashita.
Odaijini (お大事に) when some is sick you should say this. It means "hope you get better" something like that.
Sore wa ikenai ne (それはいけないね) "Sorry, that's bad isn't it."
Itadakimasu (いただきます) every time you eat you have to say this phrase. It is considered rude if you don't. It means "I accept the food"
Gochisosama (ごちそさま) "thanks for the meal"
Ittekimasu (いってきます) "I'm leaving" Homestay students should say this before leaving their homestay house.
Tadaima (ただいま) "I'm home" another must for homestay students when they return home.
Daijoubu (大丈夫) "are you alright?"
Tetsudatte mo ii desu ka? (手伝ってもいいですか) since there is no direct way of asking people if they need help in Japanese I just used this phrase. It is a little awkward because it is direct. It means "may I help?"
Konnichiwa (こんにちは) "Good afternoon"
Ohayo (おはよ) "Good morning"
Konbanwa (こんばんは) "Good evening"
Oyasumi (お休み) "Good night"
Ki o tsukete (気をつけて) "Be careful"
Abunai (危ない) "Dangerous"
Onaka ga ippai (おなかがいっぱい) "I'm full"
Onaka suita (おなかすいた) "I'm hungry"
Ojamashimasu (お邪魔します) You say this every time you enter someone else's home. Basically it means "sorry for the intrusion."
Otsumaranai koto desu ga (お詰まらない事ですが) when you give a gift, it is proper to say this phrase. It basically means "this boring thing."
O saki ni (お先に) when you go ahead of someone you say this. When you want someone to go ahead of you say "O saki ni douzo"
Omedetou (おめでとう) "Congratulations" if you want to say happy birthday its "o-tanjoubi omedetou."