Now learn Japanese or learn Korean first. Personally, I think it's better to learn Japanese first, because Japanese pronunciation is much simpler than Korean, and many Japanese characters are very similar to Chinese and easier to identify. So this aspect can speed up your study.

The grammar of Japanese and Korean is somewhat similar, so after learning Japanese, you will first find that the grammar of Korean is very simple. Secondly, after learning Japanese, you will find that many words in Korean have similar pronunciations to Japanese, and you will find many interesting pronunciations. Actually, learning Japanese is helpful for you to learn Korean.

Personal opinion, for reference only. If you are satisfied, agree. Give a point. Hey hey.