How to write it?

リンゴ
アップル
りんご
林檎

When writing apple in Japanese, You can use either Katakana, hiragana or kanji.
You can also use the English-like sound アップル in Katakana.


Advanced Tips

We usually use リンゴ or りんご to discrive an apple itself.

E.G.
私わたしはりんごが好すきですor 私わたしはリンゴが好すきです
/ I like an apple.

We don’t use アップル for an Apple itself. we use アップル for something mede of apples including an apple pie.

E.G.
昨日きのう、アップルパイを作つくりました
/ I made an apple pie yesterday.

you can see 林檎 in novels or poems. the Image of 林檎 is a bit stiff and sounds clever. So, sometimes, junior high school students who try to look like adults want to use the word.

E.G.
彼女かのじょは林檎りんごの花はなのように可憐かれんだった
/ She was as pretty as an apple blossom.

I also want to give you some information. you may see あっぷる.
I think it’s rare, but we use this when we want to express a person who isn’t used to pronouncing it. For example, in a scene where a little child manages to say it for the first time.

E.G.
姪めいの花子はなこはアップルパイを見みて「あっぷる」と言いいました
/ My niece Hanako said “ an apple” when she saw an apple pie.