Well, the general rule is that all kanjis (ie nearly all) have several ways how to pronounce them. This is true not only for 入 but for all 2000 + kanjis used in Japanese.
The basic principle is that there is kun and on-yomi .https://en.m.wikipedia.org/wiki/Kanji#Kun'yomi_(native_reading
Normally this principle is explained quite early on by your teacher. If she/he hasn’t explained that yet. Please ask her/him.
But even within Kun-yomi and on-yomi there are different ways how to pronounce. Because the kanji is the MEANING not necessarily the sound.
For 入 both “I” and “hai “ are kun-yomi. Iru 入る being the intransitive verb and 入る hairu being the transitive. As you see apparently both “iru” as well as “hairu” are both written as 入る so only from the context one can understand if the transitive or intransitive verb is meant. (Or from the conjugated version. Ie 入って “haitte “ vs 入れって irette. Same te-form for both verbs.
The general idea kun -yomi is that it is the Japanese word for the meaning. Eg 入 means “inside “ in various different ways. Like “to enter” , “to put in(side)”,” entrance”...
Eg 食 means “eating, food...”. In Japanese before kanjis were imported already a word for “to eat” existed:taberu. So once Japan started to import kanji from China they saw 食 and knew that it means “eating”. So then they used it to be read “ta “ in 食べる. But the Chinese reading (on-yomi) was „shoku „ that’s where the word shokuji 食事 comes from.
With “hairu” and “iru” in Japanese already two verbs existed that both can be represented by the idea of “inside” that is contained in 入. So the Japanese took this one kanji and used it for both verb forms. The transitive and intransitive.
Now the pronounciation “nyuu “ is one if the on-yomis. The general idea of on-yomi is that it is the “Chinese “ pronounciation. But this is what the Japanese made if the Chinese pronounciation when they first imported the kanji. Pronounciation in China has changed over time. Plus Japanese didn’t always get the pronounciation right. And some kanjis we’re imported multiple times from different parts of China. Something that in the 15 century in central China was pronounced as “ko” to Japanese ears may have been pronounced as ”kyuu “ several hundred years earlier in an other part of China.
For some kanjis one can still very clearly hear the Chinese pronounciation in the Japanese. Eg the “sha “ sound in “kansha” 感謝 is very clearly the same (well, quite similar) sound and meaning as in Chinese 謝謝 https://en.m.wiktionary.org/wiki/
But with other kanjis the on-yomi is now very far to the standard Chinese pronunciation of the kanji as the two languages have evolved for a long time.
How to know how to pronounce them?. Well, a lot goes down to studying kanjis with their pronounciation. But the general idea is that compound words of multiple kanjis are more likely to be read both in on-yomi, while a single kanji specially combined with some more hiragana has more chances to be in kun-yomi. But this isn’t a 100% rule.
It’s probably true around 90% or so (gut feeling) and 入 is a good example.
入口 iriguchi although it is a combination of two kanji isn’t pronounced nyuukou , but with kun-yomi for both kanjis.
If you don’t know the word you have no way to knowing the correct pronounciation. I remember a Taiwanese colleague of mine who speaks Japanese extremly well was surprised that 通行止 つうこうどめ wasn’t tsuukoushi , which admittedly makes more sense. Tsuu and kou are both on-yomi while tome (or the more “weak “ form of “dome” )is kun -yomi . So within the same word some kanji are pronounced as in-yomi and others as kun-yomi. But this is the exception.
I would guess that about 80-90% , if the word consists of multiple kanjis it’s On-yomi. But no guarantee for that. Plus as there are also different on-yomis due to different imports of same kanji . So you can’t really know how to pronounce it anyway.
Well, don’t panic. It will become clearer and clearer aS you study more. Your teacher should also CRA me to help you through this bump. And trust me (N2 level european) after some time you starrt to begin getting a feeling for it. Not always correct. But as a good guideline.
Enjoy studying Japanese!