After being here for about one week, I can`t help but notice some very useful and important words you should be able to say, I think those words are すみません (sumimasen) and お願いします(onegaishimas). the first one can be used to say sorry or simply excuse me and the second one means please.
Why are they very useful? because every conversation with people you want to ask help with -whether the postman, Secretary, waiter, etc- always starts and ends with those words. As a foreigner -which is evident because I look different than the rest of the bunch - to be able to say those two words seem to be a very huge amusement to them and it also shows them that you have some sort of appreciation to the culture - in instance Japanese love it.
of course there are also other words that are super useful, but I think those two words describe the Japanese the best - although I might not be credible enough to say that since I`ve only been here for a week.