我要学习汉字

totally correct, we often use '我要...' when we make up our mind to do something

因为我学习日语，我会念书和我会了解汉字。

1.Does your '念书' mean 'study' ? It's wordy in China but I don't think it's commonly used. We always say '学习' for 'study' and '读书' for 'read books'.

2.'会' and '了解' have the same meaning, but the latter is more formal and can be used in writing.

但是，我不知道如何发音汉字。

1.'发音' is an intransitive verb.So you can just say '我不知道如何发音' or use the transitive verb '读'，which means 'read', and say '我不知道如何读汉字'.

2.you use '如何' very well. It's a formal form of '怎么'. But remember, do not use '如何' in oral Chinese except you are giving a lecture.

我要学习汉字

因为我学习日语，我会念书和我会了解汉字。
但是，我不知道汉字如何发音汉字。

汉语的汉字和日语的汉字写法和发音并不一样。

汉语汉字是简体字，基于拼音；

日语汉字是繁体字，基于50音图；

写出来可能彼此看得懂一些，但是读出来就完全不一样了。