So I'm trying to start a manga series, I plan for it to be a extension of Hiroiko Araki's JoJo's Bizzare Adventure. I was wondering if I were to post it on a manga site, would it be allowed or counted as copyright. Because I wanted to do something fan made in the JoJo universe but has nothing to do with what hes made besides the concept of JoJos. I would put credit to him of course but I'm wondering If i could even post it without it being taken down.
For legal questions such as these, it is always good to contact a copyright lawyer in your country. Laws regarding fair use, copyrights and the likes differ per country, and a copyright lawyer should be more than capable of answering your questions. And this answer should not be seen as legal advice.
What you are describing is fan fiction/derivative work, both of which can be considered a subsidiary right under the original copyright, and can be tested as such. Which would require you to obtain a license to produce the work.
However, depending on the studio, producer(s) and authors the creation of such work may be tolerated. To ensure you are allowed to make such works, it is best to contact their respective legal departments prior to publication anywhere.
Additional you may want to take care in regards to which 'manga' site you upload it to. If condoned to begin with by the legal department(s), a question in regards to your distribution method may be asked. As many manga sites are of an illegal nature (piracy providers), mentioning publication to such would lead to bad blood, if not out right rejection to begin with. You can take a look at Our list of known legal providers as a reference.