The three main uses of the target particle に are as follows:
To show the target (objective) of an action, e.g. I will go to school.
To show the location of an object, e.g. the book is on the desk, the pen is in the box.
To mark the time of an action, e.g. I met my friend at 8pm.
Because its translation in English differs depending on the usage then I would avoid thinking of it as “to”, “on”, “in” or “at” in English and remember the particle through example sentences.
1. The Target of an Action
私は 電車に 乗りました。
I rode the train.
彼は 学校に 行きました。
He went to school.
私は 財布を 鞄に 入れました。
I put the wallet in my bag.
In the last sentence the English translation includes the possessive (my wallet, my bag) but neither of the Japanese sentences contain the possessive particle. The Japanese language redacts pronouns, objects, topics, and just about anything else when they are obvious from the context. In this case, it’s reasonable to infer it was my wallet that I put in my bag. Consequently it is very unnatural—but not grammatically incorrect—to write:
私は 私の 財布を 私の 鞄に 入れました。(✗)
I put my wallet in my bag.
This is one trait that tends to unite all beginners: the desire to add such details when speaking Japanese—one born out of directly translating from the English.
私は ロンドンに 行く 予定です。
I plan to go to London.
In the last sentence the verb “to do” is modifying the noun “plan”. This is a very common phrase.
The meaning of target is not restricted to a physical movement from A to B; it also refers to the direction of an action, e.g. I phoned my friend (the direction of action is from you towards your friend).
彼女は お母さんに 電話しませんでした。
She didn’t phone her mother.
Usage of the target particle with the verbs “to meet” and “to speak” is perhaps a little less obvious.
私は 友達に 会います。
I will meet my friend.
私は 先生に 話します。
I will speak to the teacher.
2. The Location Marker
As a location marker に is very often used with the verbs ある and いる (the verb “to exist” for inanimate and animate objects, respectively).
猫は 庭に います。
The cat is in the garden.
本は 机の 上に ありました。
The book was on the table.
大学は 東京の 中心に あります。
The university is in the center of Tokyo.
3. The Time Marker
に can also mark the time an event took place, e.g. I went to sleep at 10pm, I phoned my father on 4 July.
私は 十時に 朝ご飯を 食べました。
I ate breakfast at 10 o’clock.
直子は 六時に お父さんに 話しました。
Naoko spoke to her father at six o’clock.
Note how in the last example sentence に is used twice but for different purposes. This is perfectly acceptable and shouldn’t create any confusion given the context. In other words, you can’t “speak to 6 o’clock at your father”.