Mt Fuji, Mount Fuji, Mt. Fuji
proper noun
Just look at this word. What do you think it means? That's right - it's a name (Fuji) plus the kanji for mountain. Put those together, and you have Mount Fuji. In English the "Mount(ain)" part comes first. In Japanese, it comes at the end, sort of like the name ender さん, ちゃん, and so on. You can add 山 to the end of any mountain's name too, not just Mount Fuji.
ふじさん
Normally ふじ would be in kanji, but I wanted you to see this kanji use in action since it's more useful than just being used on Mount Fuji. This is treated like a jukugo word (combo kanji word). In the case of jukugo, usually the on'yomi reading is used, meaning you just have to remember the on'yomi reading of 山. Luckily, you learned this when you were learning the kanji, so you should already know it!
あっ、ふじ山だ！
Oh, it's Mt. Fuji!
エベレスト山は、ふじ山より大きいですか？
Is Mt. Everest bigger than Mt. Fuji?
いつかガールフレンドにふじ山の上でプロポーズをしたいんです。
I want to propose to my girlfriend on the top of Mt. Fuji someday.