You use Katakana if the word is not a Japanese origin. For example, the word "Night". You would say it in Japanese Hiragana (Kanji): "よる(夜)"
But you can also say it in Katakana, which is "ナイト*". Although this is not necessary as there is a specific word for night in Hiragana/Kanji. You'll probs encounter these in mangas
*ナイト can also be as 'knight'.
Well, yes and no. It depends on the context f(^_^;
Since Japanese doesn't have articles like "a" and "the" or plural nouns like we do in English, レストラン can be translated to "restaurant", "restaurants", "a restaurant", "the restaurant", or "the restaurants" (I think I got them all.) If 一 (いち) is inserted into a sentence to modify レストラン, then it can limit the possible translations to just the singular ones. You'd have to use context to narrow it down further than that.