で (de)

で most often marks where an action takes place or the means by which something is done.

Location of action

学校で勉強します。
Gakkō de benkyō shimasu. — “I study at school.”

家で食べます。
Ie de tabemasu. — “I eat at home.”

Means / method

電話で話します。
Denwa de hanashimasu. — “I talk by phone.”

Do not confuse で (action location) with に (destination or existence).