How to read kanji?

Hello
So I'm kind new on learning kanji but I don't understand how the kunyomi and onyomi system works,which way shall I read the kanji?
For example
The kanji 人
Kunyomi = hito,-ri,tsu
Onyomi= Jin ,inn
How do you know which one of the 4 shall be used?
Whenever I see a kanji of a japanese word I get no idea on how to read it,should it be onyomi or kunyomi??
Is there any rule on how to read them or is it just something you know by memorizing and becoming really fluent

Thanks

You have to just memorise it as part of vocabulary learning.

There are exceptions to the following (particularly with names) but typically if a kanji has no hiragana attached to it or it's a multi-kanji compound word you use the onyomi. When there is hiragana attached to the end of the kanji you use the kunyomi.

Again there are exceptions, people's names is a big one, but this will get you most of the way there. The context is a big help as well. But as Firas wrote, you just have to learn it as part of the vocabulary.

How do you know which one of the 4 shall be used?

Well, those are just "possibilities" of how you can read that kanji. How it's read depends on the "word," so what you need to do is to learn the word. Here are some examples;

When you want to say "a person" in Japanese, you say "hito".
When you want to say "there is a person," you say "hito ga imasu." This is written as 人がいます

When you want to say "two people" in Japanese, you say "futa-ri."
When you want to say "reservation for two people please," you say "futa-ri-bun no yoyaku o onegaishimasu." This is written as ２人分の予約をおねがいします.

As you can see, the first 人 is read "hito" while the second 人 is read "ri". They are two different words, and you can't say "ri ga imasu" or "futa-hito bun".

So, hey, one kanji can be read in many ways! What great possibilities! But they each represent different words.

What ..(guest) writes is basically correct: When there is hiragana attached to the end of the kanji you use the kunyomi.
But if the hiragana attached to the kanji is a particle like が, の, を, に, へ, there is no way to know if it's meant to be kun or on. For example, 金が(かねが:money) 金が(きんが：gold)

But if the hiragana attached to the kanji is a particle like が, の, を, に, へ, there is no way to know if it's meant to be kun or on. For example, 金が(かねが:money) 金が(きんが：gold)

Not really, because it would be very obvious from the context.

This has nothing to do with particles, either. 金（きん） and 金（かね） are just different words.

If the kanji precedes "る" or "む", it's 99% kun-yomi, right?
There are some hints on how to read without knowing the context .

related to this topic, just a glance at the map some time ago surprised me..

now I'm living in Sennan (泉南), but just next to us is Izumisano (泉佐野)
泉 - "sen" vs "izumi"

and also next to us is Hannan (阪南), which shares one character with Osaka (大阪)
阪 - "han" vs "saka"

may seem funny at the first sight, but it's not so funny when I realized that I need to learn how to read kanji.. ^_^;

There are some hints on how to read without knowing the context

Yes, but that's like saying that there are some hints on how to judge if "read" is read like "red" or "reed" without knowing the context. There's not much point to it. But again, yes, I agree. And all I'm trying to do is to make things simple for learners.

It was big problem for the native Japanese to master how to read that chinese characters.

There are many special cases but the teacher in my childhood told us students that words consists of several kanjis are read in On-yomi in many cases.

In this case, 人間　nin-gen, 人生　jin-sei, 弁護人　ben-go-nin.

If it is used one kanji and kana like 人でなし　hito denashi, 人たらし hito tarashi, mainly it is pronouced in Kun yomi.

But there are manu special cases.

Really troublesome.