に

existence / location / time

Marks where someone or something is, and the point in time when something happens.

Readings

How to form it

With いる and ある, に marks where something exists: 机(つくえ)の上(うえ)に本(ほん)があります (there's a book on the desk).
It also marks where someone lives or stays: 東京(とうきょう)に住(す)んでいます.
After a clock time or a date, に marks when something happens: 七時(しちじ)に起(お)きます (I get up at seven).
Time words relative to now, like 今日(きょう), 明日(あした) and 毎日(まいにち), take no に.

部屋(へや)に猫(ねこ)がいます。

There's a cat in the room.

毎朝(まいあさ)七時(しちじ)に起(お)きます。

I get up at seven every morning.

Using に for where an action happens. Say 図書館(としょかん)で勉強(べんきょう)する, not 図書館(としょかん)に.
Using に with a length of time. Say 二時間(にじかん)勉強(べんきょう)した (studied for two hours), with no に.

でlocation of action
Where an action takes place: 公園(こうえん)で遊(あそ)ぶ (play in the park).

にdestination
The same に with a movement verb marks where you go: 学校(がっこう)に行(い)く.