What is the best way?

1. Learning traditional Chinese may help you learn Japanese but it could affect you sometimes as well. For example, when you see 勉強 in Japanese, it means to learn sth while it has the meaning of being reluctant to do sth in Chinese. Despite this, the more things you have known, the more knowledge you can use to help you learn languages. After all, they are included in the Chinese-character cultural circle.

2. The text below are written in traditional Chinese and have simplified Chinese in Appendix. You can learn both of them at the same time.
當代中文課程(A Course in Contemporary Chinese 1~6)
新版實用視聽華語(Practical Audio-Visual Chinese vol.1~5)
實用中文讀寫（Practical Chinese Reading & Writing 1~2）
華語簡易通系列(Easy Chinese for you: Basic Chinese~ Beginning Chinese) →This one has two versions: Chinese-English and Chinese-Japanese versions
華語情境圖畫詞典(Chinese Picture Dictionary: Learn Chinese Through Realistic Scenarios)
看圖學中文語法系列(The Ultimate Illustrated Chinese Grammar Guide: Basic Level~Advanced Level)

3. Taiwanese speak Mandarin and they use traditional Chinese which is the same as Hong Kong people. But they also speak Taiwanese, one of the main dialects in Taiwan, in their daily life. Speaking Taiwanese is more common than writing it. In general, the characters in Taiwanese are from traditional Chinese but some may be only seen in Taiwanese. Some Taiwanese do not even know how to choose the right characters when typing it. That is to say, pronunciation will be more important to express your thoughts when using Taiwanese.

Well,traditional Chinese is different from Taiwanese,which is another language.
If you wanna learn Japanese latter,learning traditional Chinese would be better.

Actually some of Japanese character's letter shape is different from Traditional Chinese character.
level of complexity
↑High
Traditional Chinese character
Japanese character
Simplified Chinese character
↓Low

Do you know this App?
Pleco Chinese Dictionary
Pleco contains not just simplified characters but also the traditional ones.

https://play.google.com/store/apps/details?id=com.pleco.chinesesystem&hl=ja&gl=US
(iOS also available https://apps.apple.com/us/app/pleco-chinese-dictionary/id341922306)

Traditional Chinese use in Taiwan, Hongkong and Macao.
example :
Traditional Chinese : 後 back, 后 empress
Simplified Chinese : 后 back, empress

Both pinyin and the tone are same, but only the Hanzi (Chinese character) are different.

It's a bit hard to find Traditional Chinese textbook, only few use pinyin, most of them use zhuyin to read & type the Chinese character.

Sorry, for my bad english.

First of all, I think you mean simplified and traditional (characters). As far as I know, while most of the Japanese characters are still the same as the traditional Chinese characters and some with similar meanings, there are some differences: Japanese Kanji has two different pronunciation systems for Chinese characters.
1. simplified is easier to write and is been wildly used. Since they are in the same writing system (traditional and simplified), I think it will help you to learn Japanese later in that way. It is easy to shift from simplified to traditional. (You could learn it from a dictionary, newspapers, or a teacher who knows both, can introduce both to you.)
2. supporting my first answer. The traditional textbook has fewer choices.
3. Taiwan and Hongkong are using Traditional Characters. Taiwanese sounds are similar to Mandarin, some words are different.

If you do business or live in Taiwan, Hong Kong, Macao, or Japan, learn traditional Chinese, otherwise simplified Chinese.