で

means / tool / material

Marks how something is done — the tool, vehicle, language or material: "by", "with", "in".

Readings

How to form it

Put で after the tool or method: 箸(はし)で食(た)べる (eat with chopsticks), 電車(でんしゃ)で行(い)く (go by train), 日本語(にほんご)で話(はな)す (speak in Japanese).
It also marks what something is made of: 木(き)で作(つく)った椅子(いす) (a chair made of wood).
Going on foot takes no で: 歩(ある)いて行(い)く.

電車(でんしゃ)で会社(かいしゃ)に行(い)きます。

I go to work by train.

日本語(にほんご)で話(はな)しましょう。

Let's talk in Japanese.

When the material can no longer be seen in the result, から is used instead: 牛乳(ぎゅうにゅう)からチーズを作(つく)る (make cheese from milk).

Using に for the vehicle. Say バスで来(き)た (came by bus). に is right only for getting on: バスに乗(の)る.