N5
で
At, In
私はここでピザを食べる。
I eat pizza here.
Explanation
「で」 goes after a place and says where something is done; an action follows: 「図書館で勉強する」 (study at the library), 「レストランで食べる」. It can also mean 'by means of', 'with (a tool or material)' or 'because of', which come later; at N5 just remember 'do something at a place'.
Mixed up with
「で」 is where an action happens; 「に」 is where a thing or person exists, usually with 「ある」 or 「いる」: 「部屋で寝る」 (sleep in the room), 「部屋にベッドがある」 (there is a bed in the room).
Exception: when the thing that happens is an event itself (a festival, exam, party, meeting), the place takes 「で」 even with 「ある」: 「学校でお祭りがある」 (there is a festival at the school).