When you see a kanji in Japanese, it is typically using what is called the kun'yomi or the on'yomi, depending on context. The on'yomi is the Chinese pronunciation (since kanji originally come from China), the kun'yomi is a sort of adaptive Japanese pronunciation. You can learn about this in detail here: https://www.tofugu.com/japanese/onyomi-kunyomi/
本's on'yomi is ほん (hon), and its kun'yomi is もと (moto). In my experience, you will almost exclusively see it read as hon, but there are some exceptions. It can become ぽん (pon) or ぼん (bon), depending on context. This is called "rendaku", and it basically means that when two kanji combine, sometimes the pronunciation of the second one changes a little. There's a lot of really specific rules you can read a good summary of here: https://www.tofugu.com/japanese/rendaku/
It's hard to explain without a lot of text, but a lot of the time if the second kanji in a compound word isn't "voiced", it will become voiced. If you don't know about voicing and dakuten, that's basically how さ (sa) becomes ざ (za) when adding an accent mark to it.
If you have any questions about the above stuff, feel free to ask.
I'm going to just take a guess and say that you're asking because you've probably noticed that sometimes 日本 is read as にほん (nihon) and sometimes it's read as にっぽん (nippon). The simple explanation is "just use にほん (nihon)". The slightly longer explanation is that にっぽん (nippon) feels a little bit more official, like saying "The United States of America" as opposed to "USA". The reason why the different readings exist is really specific and historical, but if you're interested, here's an article on it: https://www.tofugu.com/japan/country-names-for-...