I agree with PECO. Pronunciation of 漢字 changes depending on 1) if it is original Japanese word to be read in 訓読み, 2) if it is a combination borrowed from Chinese to be read in 音読み, 3) if it is a combiation of more than one Japanese words, and 4) if it is a propper noun. Exaple of　花: 1) はな 2) 花粉＝かふん 3) 草花＝くさ＋はな＝くさばな; 花火＝はな＋ひ＝はなび 4) 花子＝はなこ A general rule for the combination of more than two Japanese words in 訓読み　to remember is: 1) when a word is standing alone or placed at the start of the combination, use the original pronunciation, and 2) when a word is placed at the end of the combination, change the first sound to 濁音. As PECO suggested, there may be some exceptions and you will have to memorize those exceptions just like any other languages.

Hi, Ricky 初次見面!! 我叫PECO 請多多指教!! About your question!! I think you should better "just remember" thistime is さん ､ thistime is ざん､ please remember just one word 登山=とざん AND ｓｔａｒｔ　ｐｒｏｎｏｕｎｃｅ　か行､ さ行 ､は行 ａｒｅ ａｌｍｏｕｓｔ　ｓａｍｅ　ｔｏ　ｔｈｉｓ　ｔｈｉｎｇｓ　ｂｕｔ　ｓｏｍｅｔｉｍｅｓｅ　ｄｏｎｔ　ｍｕｔｃｈ　ｔｈｉｓ　ｔｙｐｅ． for example....富士山＝ふじさん（Ｉt is proper noun), 何人＝なんにん(it is another pronounciation... but sometimes say なんびと（